Because you only care about certain buffs/debuffs in an encounter. For example, in WoW, arcane mages have personal debuff named Arcane Charge. They are constantly gaining and losing stacks of that debuff. As a healer, I don't care about that debuff. It's irrelevant to me, just taking up space, and so I would prefer not to show it on the unit frames. Also buff icons tend to be pretty small, so you have to pay a lot of attention when there are multiple debuffs. A proper set of buff/debuff filters makes a great deal of difference, in my mind.
Thing is that this is generally pretty complex to set up. I'm not really sure that you can do this in a default UI. WoW's current default raid frames in Mists are interesting, because Blizzard has designated certain debuffs in encounters as "important" and they appear larger than other debuffs. You can't edit this list, but I find it's "good enough".