The Overwatch comfortably outranges almost every Tier 1 land unit, except for artillery. It especially counters skirmisher units like Rocket Bots and Missile Trucks, while easily sniping support units like Resurrection Bots.
Build it on your front line to out-range and take out enemy laser towers.
Despite its long range and heavy damage, it is hopeless when built alone. Remember to support it with other defensive structures and units.