Houston, we have a client!

(I wish I had the time to post that somewhere more exciting! One of these days I'll find time to become active on this board again - I miss it! Just too many things going on...)
I'm still trying to brush up on my understandings of Accounting and Bookkeeping. This question is about accounting for retainers. I was surfing the QuickBooks community to find an answer and came across this:
"If you collected $1,000 retainer from your client, invoice it, but use an "item" on your item list where the "Retainer" [current liability] account is credited when the item is billed. The transaction would be:
DR: Cash $1,000.00
CR: Retainer $1,000.00
When you do work for the client, you'll invoice the work, and book the invoice to the AR as normal. However, instead of collecting cash payments against the AR, you offset it against the Retainer account via a barter transaction, thru a cash type clearing account.
Say you did $300.00 worth of work, then bill the work, and the transaction would be:
CR: Income $300.00
DR: AR: $300.00
Then do a receive payment, but deposit to the clearing accout. Note that no cash is received. It'll look like this:
DR: Clearing account $300.00
CR: AR $300.00
Then do a JE as follows:
DR: Retainer $300.00
CR: Clearing account $300.00
The result is $300.00 is used up in the retainer account, with $300.00 booked to Income. "
I'm confused - is there a reason you wouldn't Credit Income and Debit the Retainer account directly? Maybe it has something to do with the way QuickBooks handles invoicing?
Any help here would be appreciated. I'm so excited to be at this stage in the game!