Because you still want to bleach your whites. Sheets, towels, t-shirts from the gym... bleach goes all over that stuff, and it smells great! (And yeah, anyone who doesn't have white towels is doing it wrong. Eww, can you imagine all the filthy on a towel that hasn't been bleached?!)

Separate cloths by colour, and by type (glad you're doing half of that already!)... you wouldn't wash jeans with dress shirts... let the machine's auto-sense take care of the quantity / duration issues.