Many early stage companies do not have their team operating together for a long period, an accelerator generally provides them a reason to be in the same place for 3-4 months. Most programmes have a set of talks by experienced folks, providing all teams an opportunity to learn about how to grow their business - learning together as a team and with other teams in similar circumstances (early stage), helps builds bonds, learn new things, make new connections and increases the chances of either finding better ways of operating the business (ie. learning to make a pitch, convince investors, plan for the year ahead, find out who’s really going to buy what you’re making, is this thing really what you care about?)
Some accelerators take a % of your company in return for some funding (some up to $500k), give you access to a 3-4 month programme of speakers/investors/potential customers and some will introduce you to their alumni (some accelerators have a few unicorn companies) who can help in business, legal, technical, recruiting, advisory and they do that as they pay it forward.
For some companies, participating in a programme such as this will help “accelerate” the business. Some teams do break up in such environments but most people feel that it’s where they meet new friends or sometimes, they’d leave their projects and join other companies instead.