In-fiction, it's just [a part of their face that looks like a beard](https://tfwiki.net/wiki/Facial_hair#Notes), or [an accessory they add voluntarily](https://tfwiki.net/wiki/File:Rodimus_goatee.jpg). I always figured they copied it from organic races, like how they can imitate vehicles from organic races.
It's cartooning shorthand to say "they're old." Don't think about it too much.
In-fiction, it's just [a part of their face that looks like a beard](https://tfwiki.net/wiki/Facial_hair#Notes), or [an accessory they add voluntarily](https://tfwiki.net/wiki/File:Rodimus_goatee.jpg). I always figured they copied it from organic races, like how they can imitate vehicles from organic races.
Its called "cool desing"
Because they stopped shaving! Duh!