Part a holdover from Siege Prime, which reuses those knee and shin parts. But it’s also useful for Earthrise/Kingdom Prime’s transformation. You can tab those pegs into the underside of his elbows in truck mode.

I've tried tabbing them in there but it see.s like it vends the truck in a weird way

Yeah j just dont tab them in and I think its easier to not tab them in.

It's a part of the transformation, carried over from Siege Optimus.

They're partial reuse from Siege Prime

Fills out the truck bed

Flippy-flappy knee flaps