Hey!
Firstly GB usually stands for Golden Bee, I am assuming you are referring to crossing TB to create Mischlings.
My understanding for creating Mischlings is to have a backup of the important TB genetics in case of tank disaster. As the CRS/CBS Mischlings are generally hardier than TB, you can have many generations of TB genes stored in the Mischlings and then breed TB back out if you lose your original TBs.
Also, for those wanting to get into TB but can't afford it, the Mischlings are a cheaper way to get into the game.
Hope some of that makes sense!?!