You can turn it into lots and lots of blue cash! Blue xp players = balls = cash
To expand on this a bit, leveling guys will always give you a net positive with blue cash(once you trade the balls in for loyalty boxes). Each time you level a bxp player, you're essentially creating a loan with interest. The bigger the discount(FP/weekend), the bigger the interest rate.
side note: mystery box discounts also apply to loyalty boxes.
Basically what @DFBB said. Level the BXP players to get the balls and increase your cash is what I use them for. A lot of BXP bonus players in events too so it could be used for those "rental" players as well.
"Get ready for some NFTs" - VIP NEWSLETTER ca. 2022
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side note: mystery box discounts also apply to loyalty boxes.