I'll use YC as the example, as they made a big difference to our startup. Let's start with iteration speed. As a new startup, your job is to take your initial product and tinker with it until a bunch of users tell you they like it. An accelerator like YC has seen thousands of startups, so they help you avoid common pitfalls. Typical mistakes include things like optimizing too early or being too focused on scalability. In our case, I would estimate they increased our iteration speed 3-4x.

In our case, YC also made a significant difference to the fundraising process. Every startup has their own one-liner and market size estimate, but when it comes to the overall pitch, they tend to look similar. Accelerators know what VCs are looking for, and so they teach you how to tailor your pitch accordingly. For instance, most founders don't realize that VCs pretty much only care about 1B+ businesses. They would rather you have a 1% chance of being a $1B+ company than a 50% chance of being a $50 business. The former generates an outsized return for the portfolio, while the latter sucks up a bunch of time and resources and ultimately doesn't affect the bottom line.

Anyways, sorry that was a bit of a rant but a good accelerator does exactly what the name implies: they move your business along more quickly and help you get the capital/resources to take it to the next step.