Smart contractors always request a REA before they request a claim or CDA. A REA is more informal and gives the Contracting Officer more "Wiggle Room". Once the Contracting Officer issues a formal decision then it is usually set in stone. A REA gives a contractor a chance to discuss the issue with the CO.
A REA can always be changed to a formal claim. The the costs of pursuing a REA are allowable under the FAR whereas a claim is not.
I could go on in excrutiatingly boring detail. If you want to know any specifics PM me.

One thing that I didn't make clear enough. It doesn't work if the Contractor makes a mistake.

Nuke, would an REA be similar to a change order, or is the REA more for after the work is completed as opposed to during execution of work?

An REA is usually a prerequisite to a Change Order. Sometimes they work in favor, sometimes they don't. As was stated earlier, it's more informal and allows for negotiation. The REA can be requested at anytime.

I was on a large environmental and demolition project in the Pacific a few years ago. Our contract had us abating about a gazillion square feet of lead based paint on the buildings prior to demo. The government realized that abating all this paint may not be the best way to spend the taxpayers money. To make a long story short, they cut our scope by 75% and wanted to reduce our fee by the same amount. We requested a REA to cover our mobilzation costs, loss of economy of scale, etc.

The REA was requested while we were still working on the island, but the issue wasn't resolved for months after completion.

Clear as mud?

I got it, I guess I have issued these in the past, but not as an REA, just as a change order.
Thanks for the info, you learn something new every day!

Another thing to consider too is that you get interest on a claim but you don't on a REA.