The Crasher is a cheap mobile anti-air (AA) bot, that can easily take down light aircrafts, like scouts, or transporters. Always send a few with your army to protect it from EMP drones / Gunships or commander drop tactics.
Remember that it has no land-to-land weapons, so it is unable to defend itself.
In bigger numbers it can prevent bombing runs. It is able to walk underwater - combine it with amphibious units!