Guardian is the T1 long-range plasma battery. Built by all land/air cons it serves a very situational purpose.
It can be set to high trajectory for bigger damage and AoE, but lower fire rate and accuracy. This is helpful when you have to shoot over obstacles.
It can overshoot all T1 structures and units, but keep in mind that it requires a lot of metal to build. In most cases this metal is better spent into mobile units or tech advancement.
Building it to protect your base area is usually a big waste, better resort to laser turrets for this purpose. However, making it on extended front, or in range of enemy economy may end up winning you a game.
Useful on hills, for taking down strategic targets. Pretty useless when T2 arrives. Consider reclaiming it to get your metal back.