If you play with a Defense Crawler, you need to make use of structures and units together in order to reach the class's maximum potential. When you play with Support you need to be moving around and supporting your allies while collecting enough Support Powers and max out your Command Points Cap to ensure you can do as much damage as possible. However, when you play with Offense, it's different. All you need to do is to use all of your Command Points on GDI units and you have reached the limit. But make sure not to spam one unit type, as you can be easily countered. If you want to kill some air units for example and all you have is anti-ground units, you will need to Delete all your units and spend precious time getting the right units for the job, so make sure you mix and match your units depending on what your opponent is building and what is required at that time. All Offense units (except Engineer) can also be upgraded with a Blue Core, you can only see Blue Cores when you have an Offense Crawler functional on the battlefield.
It is nearly impossible to give a certain class a certain role or purpose, but based on what was said above, the Offense class is extremely good at attacking enemies. The reason for this is you don't have to wait to collect Support Powers or to build structures. You just build your army, and then you fight with it.
If you like to hit your opponent(s) hard with a fast attack group of well armored Tanks, then there is no better option than the GDI Offense class. With this class you will constantly try to attack the enemy over and over again. If you need to protect or lock down a certain area then the Defense class is a much better option, or if you constantly need to help your fellow commanders then pick Support class. Taking and attacking enemy nodes and capturing Tiberium crystals is what this class is good at doing. GDI Offense can also use its benefits of speed and agility to flank opponents, while Nod Offense prefers to use stealth. Use it well, commanders.
If you play GDI Offense, even just occasionally, be sure and check out the rest of this amazing guide now!