In the 9x and XP days it was good to be able to do that quickly, have the drivers handy, codecs and software, it was reasonable if you were working on a computer that was very slow to scan and suffered cruft anyway.
Now I agree you can just clean up adware and continue using it, even back in the days you could do this most times, with Ad Aware SE.
Adware is typically not sosphiticated malware.. If you have a strong infection, it's where you can consider format/reinstall. You may have both a virus and the antivirus watching every I/O, making the computer very slow, scans take hours (I've seen this at least on XP)
With particularly high end malware such as ZBOT, you may have to format/reinstall and delete every single executable file on your hard drive (including .dlls)
The PC was mildly modern (Pentium M, 1GB ram) but dog slow (rather felt like some piece of crap celeron 466)
After the aforementioned very long scans (plus one done in safe mode) the ZBOT triggered a countermeasure that infected damn everything.
Of course, that's the worst case. But when dealing with computer security, if you don't know better you assume the worst case so I can see a rationale for a blanket advice "format it". To which I add, "delete fucking everything" : driver/software/codec installers, and all your games.
