This just baffles me.
People will come through the drive through at the bank where I work, send in a wad of cash, and tell me to deposit it into their account. When I ask them to verify how much cash I have, there are some who shrug and say, "Uh...I don't know" and give a little laugh.
I don't think they realize what a horrible idea that is. What if they have $300 there and I decided to take $100 for myself and deposit the $200? They'd have no idea. That's why it's important to verify how much you have, so you know how much should be deposited. Not that I would take their money, but it's a precaution for them for me to ask and for them to know.