Because you will carry over any noise or garbage from the previous installation. Been there done that and figured out back in 95 to 98 what was best. Backup your stuff and do Full install where you format the hard drive wiping all trash from it and get a brand new system.
You can do that even with an upgrade disk and proper Product key of course.
Backup your Pictures, Music or whatever to an external USB drive or second hard drive.
Quit using MS created Folders like MY Documents, My Pictures. Use something on the second drive and start saving everything there. If your system gets a virus or whatever most times it won’t affect the second drive.
Best part of this is if a virus or whatever gets your main drive then you can re install and still have all your stuff.