There are a number of reasons for using a lawyer's services before claiming your lottery winnings. Some examples:
Only a half a dozen or so states allow lottery winners to remain anonymous. If you put your winnings into a trust, only the name of the trust becomes public.
Perhaps you'd like to avail yourself of a bypass trust which automatically names the surviving spouse as the beneficiary upon your demise and reduces your family's tax obligations.
Perhaps you were part of a group that won shared in the prize. Since only one entity can claim a lottery prize, establishing an irrevocable trust in the name of the winners will ensure fair distribution.
Perhaps you'd like a blind trust - where the trust claims the prize in its name and invests the funds for you (professional money management).
And no, the lawyer does not get 1/3 of your winnings unless you're foolish enough to agree to sharing. You pay their hourly rate and it's peanuts if you win something like the Powerball lottery.