A nice use of them is the Sawman-beacon strategy. It basically consists in building three signal fires near your base, which will regularly spawn three batallions very quickly (six if you have the spellbook power Assistance in the Time of Need)
This allows you to not worry about investing resources in early units, but rather on upgrades, heroes and basically starting your economy
Keep in mind that this strategy should be used to gain some time, because if your opponent outspams you or reaches his Lategame, and your economy isn't good, or you don't have upgrades/heroes, you've basically lost