It’s to do with melee blocking. If raiders go through more than one door they don’t go into collision mode, which means they all pour through in a clump and force colonists defending the doorway back into the base, which can be fatal. That means it is not safe to use the entrances in the walls for melee blocking in this base, as thry have multiple doors. In the towers, however, there’s space for colonists to defend a single doorway safely, if they defend the outer door of the tower. There’s still more doors behind them, however, meaning the base isn’t protected by just one easily broken door.

Hope that makes sense - it’s hard to explain about collision as it’s quite unintuitive.

Edit: also I think it looks kind of cool.