Why must you look over your shoulder before crossing the doted white line?

It really annoys the crap out of me cause my driving instructor tells me to look over my left shoulder for a split second before crossing over which is completely pointless and makes me nervous cause my eyes are looking at the road. I have no time to look over my shoulder when i have my mirrors. Someone could jump into the middle of the street and get killed cause I'm looking over my shoulder for stupid pointless "SPLIT SECOND"