Most accelerators exist to jump start pre-seed companies and get them to an investable state. The percentage of equity required covers the substantial risk inherent in such early stage companies. The small amount of cash normally offered is usually backed with marketing, design, and legal services. Office space is usually included. Accelerators normally also have an investor network and provide a demo day to show off in front of said network.
If you have all this already, can bootstrap it or fund it yourself, then by all means please do. But for *many* startups, an accelerator can be invaluable.