And the rest generally fix bugs and improve stability.
You would not believe how many people contact me complaining that their Windows machine is unstable, but when I ask when was the last time they ran Windows Updates, they say, "Oh I stopped allowing those Updates, they annoy me so I turned them off".
Fools. Just sayin'...
In W7, I found some svchost.exe instances which took up an extraordinary amount of memory. Looking at the process tree (and checking on the web), I determined that the instances with the biggest footprint were linked to Windows Update.
I’m running 8.1 now, and it’s not nearly as much of a problem. Fewer instances, less memory, less CPU time.
Caveat: Maybe this is now improved; maybe I haven’t been running this machine long enough to accumulate the cruft of several years of updates.