Windows 9x suffered from DLL hell. So every time a program was installed it potentially overwrote dlls with a different version often older or incompatible. Windows 2000/XP just redirected the installer's dlls into a per program location preventing this which is a large reason those versions were so much more stable.
Most people recommended a complete reinstall every 6 months well through the XP era but I found this was hardly ever necessary after I switched to 2000. Conversely, during my 98 days I never had to schedule reinstalls, Windows had rotted apart by then forcing me to do it!
I definitely remember the DLL hell experience that manifested as an older 2d game overwriting some DirectX dlls in the OS with older versions, and suddenly all my FPS games stopped working.
That was a fun one to troubleshoot as a 12 year old kid.
Most people recommended a complete reinstall every 6 months well through the XP era but I found this was hardly ever necessary after I switched to 2000. Conversely, during my 98 days I never had to schedule reinstalls, Windows had rotted apart by then forcing me to do it!