Your drivers license contains your address. Having this helps ensure that your purchase gets shipped to the correct location, as it is easy to transpose numbers or hear the wrong street name over the phone, that's all they want it for.
This is the main reason we ask for customers DL in the store because your address is on it and we don't have to get you to say it out loud in front of other customers who are nearby. It also speeds up our sale in the store and allows most customers to walk around without being glued to the counter to answer questions. Most of our sales people are just used to asking for DL in person and ask for it for mail order as well so the address that the item is shipped to is the same that's on the DL (in most cases). Phil.