Completion of a book

You're close. Except not promotion. That's a post-publication issue, and is usually handled by the publicist at the publishing house.

Some agents (and ONLY some) will assist with making editorial suggestions prior to subbing it to publishers. Others will just send it out as is.

When you query an agent you are basically seeking representation for your work by someone familiar with publishing houses. If you want to self-publish and do all this promotion on your own, you are welcome to give it a go. If you hope to have someone who is a little bit more knowledgeable than you, who knows legalities of publishing, etc, plus the details of commercializing your work (potentially) then it's best to find a reputable agent who has sales under his/her belt. You want to find someone who not only has had dealings with your particular genre, but has made legitimate sales on said books. (Should have a website or client list they've repped somewhere online). You also want to find out their specific guidelines for submission before you submit(usually found online, also). If your work is really terrific, your agent may even place the manuscript up for bids at an auctionhouse to get the highest bidder to publish it. Hope this answers some of your questions.

You can also hang out in Ask the Agent and read threads in forums like Goals and Accomplishments where *some* people outlined their agent/pub process.