Okay, I think the explanation is pretty obvious.

It's a stylistic choice since having a normal bot image seemed boring to me, and it's easier to get confused on which bot is which.

But there's also a reason why these exist.

(The ones that have the logo of their respective series are obvious, so they won't be included here.)

And please do note that these do not apply to OCs since they have their respective borders.

So.

Sonic: Normal bots or just regular releases

Fleetway Super Sonic & Metal Sonic: Made for events like Christmas or Halloween or other occasions. Fleetway is the only one used atm.

Sonic.exe & Kolossos: Different Icons, same use. Made for bots with Dead dove included in the tags.

Now the question is, Why sonic? Idk, I just think it looks cool as a design for avatars.

There are some avatars that don't have these borders for the obvious reasons, it's either I didn't feel like it or it doesn't fit the theme of the avatar at all, mainly for Avatars that were drawn by me.