Originally they were intended for local residents whose properties were too small to have their own garage, but now they might be rented for other purposes. I've seen local building & construction firms store spare lumber in a lockup, as well as hobbyists who keep a sailing boat or occasionally-used motorbike safe. Lockups are even used occasionally to store the corpse after a particularly gruesome murder, though that's very much one of the less common things to put in them!
The outside walls are also taken from the Armoury kit. In this case, I've chosen to resize a plain brick texture for the sides and rear, though it would be entirely appropriate to have graffiti on some or all instead.
The front wall is probably the most heavily-modified piece. It started as a plain brick wall, but I've added 3 door frames from the Armoury's garage, somewhat resized. However, I wanted to make this block look old and tatty and so I didn't want the doors themselves to be identical. 2 are from the Armoury kit, though one has been lightened up a bit. The third door is a texture that I found on the internet and resized & cropped appropriately. I found that searching Google for "metal siding" was the best way of finding such images, though even then it took some effort to find exactly what I wanted.
The sides of the roof are covered with a pattern from a Worldworks kit (can't remember which), somewhat extended and reshaped. I think they were some form of roof edging originally, so they work well for this purpose. The roof itself uses more "metal siding" textures.