Why are there vents on the outside of lockers?

The vents are to allow air to circulate through the locker. This is mainly to remove odours from the locker, that their contents usually generates.

Some lockers only have the vents at the bottom, but the better ones like these - http://www.action-storage.co.uk/Lockers/Standard-Lockers/c-1-115-141/ have a vent at the top and bottom, to allow the air to evenly flow throughout the locker.