Since the player's companions or followers may travel between maps, it is not possible to refer to them by their id, when targeting them using scripts. If a bot is to become the follower of the player, it can be targeted by it's id number until such time, as the player leaves the map. After that, the only way to target it is to use the setCompanionAsTarget call, which takes the companion's name as a parameter.
Targeting the player's companions with setCompanionAsTarget has one shortcoming, in that if the script target's the companion, and waits to do whatever it does to it, and the player leaves the area while the script is waiting, the script may not work as intended when the player comes back. That is because internally the waiting script's target is saved using the companion's temporary id number, and that may change when the player returns to an area. Therefore it is best to not leave scripts in the waiting state when using setCompanionAsTarget.