You're right about properly describing the goods and services, but that's not the main reasons. Here are some:
(1) Advice on the strength and registerability of the mark. Among other things, that means looking at similar marks and advising you about how they impact your application.
(2) Dealing with office actions from the USPTO -- if, for whatever reason, they don't like your application, the attorney will know how to respond.
(3) Dealing with oppositions -- a few months after you file your application, it is made public, and people can object to the mark. Your lawyer will know how to respond to those people.
(4) Administration -- just tracking the registration through the process, keeping track of when responses have to be made, etc....
(5) Dealing with foreign counsel. If you're trying to register the mark outside the US, a good trademark lawyer will have a network of foreign associates that they use to prosecute your mark in those other countries.
It's certainly possible, and not very difficult, to register a trademark yourself in the US with the USPTO's "TEAS PLUS" application, as long as nothing goes wrong. When/if something does go wrong, that's when you want a lawyer.