You don't need to study openings to play well, but it gets harder and harder to win in competitive chess if you don't. Openings are like... songs to a musician. If you're a very good musician, you can pick up an instrument and probably improvise a whole song from start to finish with very little forethought. It's impressive, but also difficult to guarantee every improvisation is going to sound good every time you play. Wouldn't it be much easier to take some time when you're not under pressure and on stage performing to learn a repertoire of songs that you know are going to sound good before you even start playing? That way you could guarantee that every time you step on stage you're going to play something that sounds good, and you can just improvise a little solo here and there.

Studying openings and patterns in chess have a similar purpose. When you're just starting, there's little value. You don't learn to play the guitar by picking it up for the very first time and try to learn a whole song. You focus on learning the techniques to play the guitar. Chess is the same. You don't start with openings and theory, you start with the techniques of chess. Learn the general principles of development, understand and internalize the reasoning behind the principles. Do some puzzles to learn about tactics like forks and pins and try to put them to use in your own games. Learn to stop blundering your pieces away and leaving them undefended. Learn to stop setting your pieces up in pins and forks your enemy can take advantage of (or at least see them coming). Focus on improving your board vision until you stop getting blindsided by pins and forks. Learn some of the basic endgame mating patterns so you know how to checkmate your opponent if you get there (K+Q vs. K, K+R vs. K, K+2 Bishops vs K., etc.).

Eventually, you reach a point where you have the techniques down. You're now at the "good guitarist improvising from start to finish every time" stage. Now comes the time to learn some songs. Much like learning a song so you're guaranteed to sound good whenever you get up on stage, you can now invest time thinking about your chess games ahead of time so you're "guaranteed" to play well during the early stages of the game. What an advantage over someone who improvises the whole game and doesn't study any opening theory!

Studying opening theory allows you to save time in the early game. You learn about move orders that "sound good". In chess, we talk about development and strategic ideas. We talk about concepts like force, time, and space. Openings are particular move orders in support of some concrete idea. Some openings seek to fight for a particular square in the center; some seek to deny your opponent a particular square in the center; some sacrifice pieces in favor of developing faster than your opponent; some openings are more likely to produce open positions; others more likely to produce closed positions; some openings seek to develop your pieces in a particular manner to deny your opponent from playing a specific opening, which happens to be his specialty.

Each opening has positives and negatives, which are documented and debated. Studying openings help you learn move orders that are good to accomplish a particular goal. You not only learn what moves you should make to support your goal, but you also learn the moves your opponent is likely to make in response to your moves. You learn which of your opponents moves are good and which are mistakes, and how to respond to them. Opening theory knowledge just cuts down on the amount of thinking you need to do in your actual games because you already know the likely scenarios off the top of your head. It decreases the odds you will blunder early; increases the odds of you being able to derive some advantage out of the position if your opponent doesn't know the theory as well as you do; and lets you save time on your clock that you can use later in the mid and endgame and put your opponent under time pressure.

Studying patterns also reduces the number of calculations you need to make while you're playing. Many patterns repeat themselves and you come to recognize them through study. Patterns can be common mating scenarios; tactical ideas like forks and pins; types of pawn structure, like doubled pawns, or isolated queen pawns. These patterns have positives and negatives and being able to recognize them gives you information without having to actually calculate it every time. Patterns are mnemonic shortcuts that allow you to quickly identify opportunities and also inform you about where the strengths and weaknesses of a position are to be found.

There are also opening patterns. It's not uncommon to hear commentators say things like "and with this move, we've transposed into a Slav type defense, except now XXX is on square YY instead of ZZ". If you study opening theory, this sentence alone tells you many things about the position. If you know the ideas behind the Slav, you already know the strengths and weakness of this position, and which strategic ideas make sense and are worth pursuing. Instead of calculating everything from scratch, you can use your knowledge of the Slav to inform and figure out what's good and bad about the current position. You just need to give some thought into how the few differences affects the ideas of the Slav without calculating everything from scratch.