They'll send out query letters by email or post; they'll print up your submissions package if you want to send it by post; they'll provide a list of agents and editors for them to go to; they'll even write your query letter for you if you're happy to pay extra.
There are plenty of things they don't do for you, though. They won't target the best agents and editors for your work; they won't write you a mouthwatering, irresistible query letter; and they won't be able to show you that their service is effective because in most cases, a query sent via one of these services won't even get read. You don't believe me? Then perhaps you'll believe some well-respected industry professionals.
From Jessica Faust at Bookends Literary Agency:
I stop reading when... the query is sent through a query service, your husband, your grandmother, your daughter, your lawyer, your doctor, or your dog (and yes, it has happened). Nearly instant reject. I’ll read the query, but I go in skeptically and you darn well will have to knock my socks off and throw them across the room before I can be convinced you can actually do the rest of the work on your own.
From Janet Reid, Literary Agent:
These are the very few things that make me stop reading: 1. The query isn't from the author. That includes query writing services, cover letters from your assistant, free-lance editor, etc. You have to write and send your own
Bolding mine. There are plenty of other similar comments on the internet if you want to go and look for them; but I've not been able to find a single agent or editor speaking out in favour of them. Don't waste your money: write your own submissions and send them out yourself, and remember: there are no shortcuts worth taking.