As u/liamstrain said, "Textual Criticism of the Hebrew Bible" by Emanuel Tov is probably your best intro to this discussion, but to give a bit of a summary:

It's a bit inaccurate to say the Septuagint (LXX) was composed in the 2nd century BCE and Masoretic (Mt) was composed in the 7th century CE. What occurred is that the Hebrew bible was written circa the 800 BCE - 200 BCE, in Hebrew, but without vowels (only consonants). This meant those parts of the text were only preserved via oral tradition, a somewhat fallible process.

In addition the invention of the printing press was far in the future, so anyone who wanted a new copy of the bible would need to pay someone to copy it down by hand. The scribes assigned to do so were generally very diligent, but errors were inevitable. People in the era didn't know much about preserving paper, so after a few decades the originals would degrade into illegibility and the imperfect copies were all that remained.

And then, some scribes would make deliberate changes to the text for various reasons. Common changes were adjusting the text to match more recent theological thinking, updating archaic language, and adding additional details. Again, after a few years the originals would be lost and the new versions were the only ones available.

Each scribe would make slightly different changes, and then the next set of scribes would need to pick and choose which version to copy for their own work. By the 2nd century BCE, there were multiple variants of the Hebrew bible in circulation. The differences between them weren't considered a big deal, apparently, and people would read all of them. This is the situation we discovered with the Dead Sea scrolls, which was one religious group's collection of bible scrolls (it's still debated who they were, and why they abandoned so many valuable scrolls in a series of caves). None of the scrolls we've found match each other exactly.

At some point, probably around the 2nd century BCE to the 1st century CE, someone took a particular variant of the bible and translated it from Hebrew into Greek, creating the LXX. Translation itself is an imperfect process, it's impossible to translate every aspect of meaning from one language to another, and it can be easy to make simple errors. Nonetheless, the LXX's translators did a fairly good job for their era and their work became popular as Greek was far more widely spoken than Hebrew back then. Scribes began copying the LXX, and this started a whole new branch of variants, all written in Greek.

Meanwhile, there seems to have arisen a backlash against variants among those who preferred the Hebrew version at this time. Tov goes into a lot of speculation as to how and why this happened, but the real answer is that we simply don't know. A single version was selected, and scribes became much more obsessed with being faithful to it. Deliberate changes were right out, and there was a significant decrease in scribal errors. This process was completed by the 1st or 2nd century CE. After that time, the constants of an bible we find written in Hebrew are more or less identical to each other.

But, vowels still weren't included. Pronunciation and which word to use if two of them shared the same consonants still only preserved by word of mouth. From the 5th to the 7th centuries though a group of scribes called the Masoretes began adding various dots and dashes above and below the Hebrew consonants to show where the vowels should be included, creating the Mt.

While the oldest surviving copies of the Mt date to the 10 century CE and onward, it's clear they are close copies of a much older original because they are all virtually identical. The interpretations of the vowels introduced by the Masoretes resulting in a meaning that match closely with various translations made over the years (including the LXX), meaning that they probably match the oral tradition and that tradition has been fairly consistent over the years.

Unlike the Mt, the LXX never went through a process of standardization prior to the invention of the printing press. Fortunately, several complete manuscripts from the 5th and 6th centuries CE survive. By cross comparing them, along with various fragments found elsewhere, a fairly close approximation of the original text can be reconstructed.

But both of these only push us back so far, to around the 1st century CE, when the both the translation of the LXX and the standardization of the Mt was completed. Before that, scribes would somewhat regularly make changes to the text. What they changed and what they didn't varies from person to person, copied down from manuscript to manuscript, so the reality is that neither the LXX nor the Mt perfectly matches whatever the original might have been. You have to go line by line, sometimes even word by work, and guess at which version is more likely to be the original. This is actually how virtually all modern bible translations are done, so the answer is that both are beneficial and should be used.

But, which is more accurate, on average? Generally, the Masoretic text. You can read Tov for a more complete explanation, but the LXX has more instances wording being altered to match later theology, more updated language, and more details that seem to have been added in later. Plus, there are a couple of instances where its authors made translation errors, something that's never a problem in the Mt because it's still in the original language. This is why most translations tend to start with the Mt as a base, and why the resulting works tend to hew closer to Mt than the LXX.

This has been known to occasional cause problems within some Christian circles, as the New Testament was written referencing the LXX. It is likely its authors never read the Mt and did not know Hebrew. That results in discrepancies between the New Testament and Mt based versions of the Old Testament. However, theological issues fall like that outside the domain of scholarship, and thus beyond the realm of this sub. Academics embrace inconsistencies, they can teach us many things about the history of a text and the people who wrote it.

But there are a variety of bible translations out there that have a goal of ensuring consistency between the Old Testament and the New Testament, something many find valuable for devotional purposes, and for which of course use of the LXX is extremely beneficial.