The purpose is that it's cheaper to pay a handful of well qualified, non-training-risk folks a "blended" rate to sit right seat than to pay everybody on property a retention bonus like Endeavor. It's just a recruiting tool. And it gives you (and skeds) the flexibility to pick-up (or assign) left seat work occasionally.
See it for what it is - a great (relatively) starting wage for a regional lateral move - and it's a good deal. If you take it for all that TPIC, well, you're likely to be disappointed like the many previous posters on this thread.