Actually the story goes like this, they are built by the minions at the shopkeeper namely acolytes,peons,peasant, wisps and a new minion which is a blood elf , because they dont have enough resources they initiated war with the other faction so that they can fund more for structural use, like what the US is doing in iraq soil, they invade so they can have their resources but in the meantime they have to fund themselves by buying weapons and spending money so they can invade swiftly, back to dota, so in the meantime they sell weapon and items to gain money to make use of what they have, maintaining and repairing the structure which is already built, that explains why some towers and barracks heal themselves in time, while waiting to build more structures and expand their land they wage war with the opposite faction and make full use of their time, hence DOTA! Either you are glorious and contribute to your kind or die protecting your belovely land.

So in a nutshell dota teaches you to be more patriotic, that is why badawi encourages us to play it.