One of the major annoyances with Microsoft, MSN, hotmail etc., is a continual stream of "stuff" with grandiose cute names, verbose ADVERTISEMENTS that tell you that it's WONDERFUL but with absolutely no clues to what it is, what it does (to you) or why you really would want it cluttering up your machine.
So far as I've been able to determine, WGA is just another "buzzword" name for the process of automatically validating your copy of Windows as "genuine" so that you can get automatic updates.
Once the working validation method is on your macine, your Windows license is checked each time new updates arrive; but as long as you have a current (or reasonably recent) validation you shouldn't see it happen. If you don't have the current tool installed, you may either a.) not get updates you need or b.) have to jump through additional hoops to validate and get them.
The "widget" to take care of it all will download automatically with automatic updates, but requires your permission to install it. The request for permission may or may not tell you that it's WGA, and occasionally other stuff asks for similar permissions (also often without telling you what you're giving permission for).
IF YOU GIVE PERMISSION, you usually will be asked to agree to a EULA, which is the main reason for asking permission. With a bit of cleverness on your part, the EULA may reveal what the "utility" in question is, but often it's just a "boiler plate" legaleze POS.
Since you do want updates, and "validation" is necessary to get them, using the "current method" probably gets you the least hassle with keeping Windows up to date, so you probably do want to install WGA. It's the latest validation method (at least this week).
The link offers several places where you can poke about to try to figure out the details, if you're so inclined. There may be a "higher level" (more generic) WGA/MGA page but it's likely to be 100% sales-speak.
Good luck at finding any really useful info. It's still Microsoft.