Did you ever add your white tee to a load of bright, coloured clothes by mistake? Did your heart shatter after seeing it turn into a whole different shade of colour? Yes, that is precisely why you must sort your laundry before mixing them in the washing machine.
You could have avoided the problem if you had spared a few minutes to separate your clothes. Sorting decreases the likelihood of a coloured garment bleeding over the others in cases where you control the temperature of washing.