Crossbow is a cheap mobile anti air (AA) bot, that can easily take down light aircraft, 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 not able to defend itself from ground fire.
In bigger numbers it can even prevent bombing runs. It is able to walk underwater - combine it with amphibious units!