Point Defenses, commonly abbreviated as PD, are static turrets designed to repel attacking land and sea units within range.
Compared to mobile units, PDs are tougher and usually have more powerful weaponry. However point defenses can't attack submerged enemies nor air units, making it vulnerable to bombers and gunships, as well as artillery and missile launchers that outrange it. Walls are a great combination for PD positions as the high firing position would allow them to fire over the barricades while the raiders would find it hard to break through whilst under heavy fire.
Nonetheless, PDs make for a good base defense when focusing on an attack but take heed as a single point defense won't hold out against a dedicated assault so grouping them together makes for a greater chance to defuse hostile intrusions before they traverse deeper into the base.
They however shouldn't be built adjacent to each other because it'll make it easier to be dispatched by AoE attacks, especially Mobile Artillery
All factions benefit from using both T1 and T2 PDs together.. For Cybran, UEF and Seraphim, using T1 PDs in addition to T2 PDs brings up DPS, whereas for Aeon it allows for lower overkill, taking care of T1 armies (and other units with low health) much more effectively.
Even the UEF T3 Ravager should be built with T2 PDs assisting it, due to its 8 second reload time. As Ravagers also require energy to operate so if there isn't enough Power then having auxiliery T2 Defenses can help. Though you can build power generators next to them to help offset the operating costs.