That’s a good question, and Ragnar is also covering it in the article above. Most importantly, a good accelerator is an excellent place for networking— for exchanging ideas, finding investors, mentors, valuable points of contact, and referrals. Another big reason for joining one, is to gain momentum for growth. A good accelerator pushes the team to work fast and really take down barriers, while offering fresh know-how.
If the startup is content with their current position, and has now wish to aim higher, indeed, there is no need to join. For us it was totally worth it though.