We are humans, not machines, as such we tend to be forgetful, or cannot predict things we will need by the end of the day.
I usually do groceries on my way back from work and have no room for bulky re-useable grocery bags, especially the cooler bags for frozen items. Sometimes I'll run an errand on my way to work and cannot store baby diapers on my desk either.
There is lots of stuff I keep in my car that make car sharing not desirable: tissue boxes are always full (allergies), extra pairs of sunglasses, hats for very sunny days, sun lotion, gum, water.
I also forget my phone or wallet once in a while in my car, I just take the elevator to get it back