The Tech 2 shield generators are a cheap form of base defense structures. They are designed with a shield radius to only be able to protect nearby small structures or units.
These shields are designed to defend base from early bomber runs or land rush to protect strategic structures, such as extractors, power generators etc.. Although it has less Shield points than T3 Shield Generator, you may consider building more of T2 SG for protection rather than T3 SG. T3 SG cover a wider area, but also consume a lot more energy than T2, and a drawback for T3 SG might even be it's range, since most gunships will be able to fly in and destroy it. If you need heavy protection, against large scale attacks upon your base, consider building both shields - T2 and T3, with T2 covering T3 shield, that way your defenses will be protected more effeciently, since breaking the shield down will only reveal your firebase structures (considering, that you built it for defenses) and save T3 SG from being destroyed and give time for it to reload.
It is recommended to always build these shields next to tech 2 power generators, as it will both protect the vulnerable generator, and reduce shield costs. These shields are not very expensive to build, and costs about the same as a one or two tech 2 land units. However, they consume between 100 and 250 power, or between half and one fifth of a tech 2 power generator.
The Aeon Shield of Light has a smaller shield radius, but it is stronger, reloads at a much faster rate, and consumes only 150 power. However, it can not be directly upgraded to an Aeon heavy shield.
The Seraphim Atha is clearly the most powerful shield, being tougher than the Shield of Light and possessing the largest radius, but is also the most costly.
The Cybran T2 Shield Generator is a special case. It is by far the weakest of all four shields, however, costing only a fourth of the others, and with a consumption of only 100 energy, it is the best available tech 2 shield. The ED1 can also be upgraded if extra defense is required; however since it has no T3 version it must always be built at its minimum size then upgraded. See Cybran T2 Shield Generator for more information on this shield.