Simple enough: why are you as strong or as smart as your profile lists you to be? Why did you choose to ally with one faction of Cybertronians over another?
I don't speak for anyone else, but I'll guess it was very common to base your stats on how you perceive yourself. Initially all my numbers were too high, so I reined it back a bit and admitted, yeah, I'm not very fast.
The notion of strength and intellect and endurance I all feel accurately reflect me. I wanted to have a decent pool of stats, so I gave myself a decent rank. Given my tendency to run off half cocked into certain danger without much foresight, I figured, yeah, I can have a decent Courage score. As I don't particularly care for firearms, I decided I didn't need much in my Firepower stat, and as for rank, that's entirely ego. I'd hate to be lower than a squad commander.
My faction I actually agonized on quite a bit, as I couldn't really decide what kind of Decepticon I wanted to be. I knew I would be a Decepticon from the very beginning, but where would my overpowered self insertion character fit into the canon and the timeline? Was I there from the Ark landing on Earth? Keeping Blaster down on Cybertron?
I chose G2 because the decimated Decepticon ranks made it a little easier to stand out. Being as narrative-minded as any other fan fiction author, I constructed a lengthy backstory to explain all this, and used the stats I'd filled out to come up with my bio:Decepticon Nightweaver
Nightweaver insists he is the sole surviving fragment of a human personality, imbedded into Decepticon programming. He cast out his standard issue firearm for battle blades, claiming a sudden distaste for the "ease" of a gun. His reckless charges into battle have led Decepticon Leader Bludgeon to take notice of him, and led to Nightweaver's promotion to squad commander. The Decepticons under his command are wary of following him into battle, as Nightweaver's idea of "strategy" consists entirely of rushing headlong into battle, no matter the odds, hoping to overcome the Autobots with sheer moxie.
In retrospect, not very G2-ish. I should mention how awesomely "metal" his blades are and how very stealthy his cybertronian jet mode is.
Thus, my allegiance to the Decepticons, where anyone can become leader, and I'll get there by a combination of a battle blade to the back of Bludgeon's head and sheer moxie!