Of course, the level of expertise on this forum is way above my head, but perhaps the best example of a need to scrub a logical drive came from my many, many hours of trying to cure a sound fault on a Foxconn MB.
One of this forum's stalwarts - I'm sorry I can't remember who - suggested (words to the effect) that there might be some detritus left with the different kinds of drivers.
I'd deleted existing drivers several different ways, and battled against them as they reinstalled via W7 into what should have been a driver-less environment, but to no avail. I seriously thought of switching off the substantial sound system and buying a simple sound card, such was the extent of my defeatism at that point.
For some reason, (getting the partition size to 1 gig courtesy of XP disc management I think.) I reinstalled W7, having done a long format on C
Clean disc, clean install. I didn't give the sound a thought . . . until it worked perfectly.
I guess when an OS is built with a mass of discrete files, it's not that surprising to find that an application embeds itself into a sizable selection of them. But I wish they wouldn't.