Ability to contract
People of all ages want to buy products. However, children and teenagers are not allowed to engage in all contracts and enforcement of contracts can be limited. As such, asking for the Date of Birth allows not to contract with them based on their limited ability to form contracts.
Identification
Assume that there are 2 people called John Doe in a town. The chance that they are born on the same date is small. Together with address, this can lead to a positive "This but not that person" in case of litigation.