primarily so others following you know its the right you and not an impersonator, which makes it far better to do yourself with a domain you control.

different nip05 providers charge different prices.

some free ones

nip05.social

bitcoiner.chat

nostr-check.com

nostrfun.com

or buy yourself a domain at low cost (namecheap, namesilo, etc) and follow steps at https://nvk.org/n00b-nip5