Can someone tell me what those 4 rubber grommets (2 on each side) along the gunnels are for?

The hold one end of the armrest down. The armrest swings to the back hole for the armrest to be used in the tandem position. Where are your armrest pads?

It's a solo but I think I understand now.

My solo has the back 2 also. I am not sure why, I use it to put my cable lock through.

I took my armrests off since they are basically useless, and tended to snag flyline. Also removed those little rubber grommets so water trapped in the recesses could drain out.