Auxiliary gem inlay

I'm still missing the point to auxiliary inlaying legendary gems. Since you don't get the attributes and bonuses of the main gem placement, is it just for resonance and combat rating? Is it best to put a lesser gem in first and then the ideal gem in auxiliary?

I spent some time transferring gem power because I thought the dormant gems could still be used as secondary gems. At least, thats what I understood from the info they provided.

Im pretty new and theres a lot understand about this game so any insight/ advice is greatly appreciated!

Thanks in advance!