The Blaster is a Turret in Revenge of the Titans. It's the most basic turret and available at the start of the game. Damage output of the turret is fairly low, capable of killing only the weakest of Titans in one shot. This limitation makes them useful for only the early part of the Earth missions ; once Moon is reached they should fade in favor of more powerful tower.
They may seem cost-efficient, but to kill late-game titan you need an awful lot of them. It mean inefficient add-on (since add-on have a maximum range) and difficulties to avoid titans going on them, since titans will take a long time to die and will jam or otherwise go astray and attack tower. Also, late-game aggressives titans need to be killed pretty quickly.
As a positive note, they does not create Threat for titans, so Titan will never try to avoid them or change route because they pass right before a bunch of Blaster. That help make the beginning of the game easy, but it's clunky to take advantage of after.
look up secret