Addons are mods, and most servers have them to make the servers have more content, things like characters, maps, and, well, horns.
Also in kart's case, even if you join a vanilla server, they'll probably add bonuschars, since they still consider it vanilla as the kart krew team both made it, and it comes with your installation.
Every server though has different mods active, if you dont have all of them you need to download them, you cant do anything about it aside from simply playing a lot to incrase the chances of you having a specific mod.