Because back in the ye old days, there weren't many DPS checks beyond your healers running out of mana. And before you go all crazy on me about ye old games, I've been playing these since UO.
I prefer having a parser because it allows me to see where I need to improve, and who, in the raids I lead, needs to improve. And how else would you find out you need to improve without accurate data? I can say "We need better DPS for this DPS check" all day but no one is going to always own up and say "I'll look to adjust my gear/food setup, and my rotation".