Curious why there is a derailleur, but seems to be only one fixed gear at the back? Guess you trust that motor to have enough torque to not need any rear gears?
Adding to the other comments - chain tensioners look like derailleurs, but are much simpler/can't move the chain to different gears
on rear suspension bikes with a changing chain length you need something to account for that change in chain length so it doesn't go floppy at certain points of the rear swingarm motion.
It's probably used to keep the chain under tension when the rear suspension moves around.
On a full suspension bike like this, the distance between the rear wheel and bottom bracket changes when the suspension works, so you need a way to compensate for this.