They are saving on allocation cost, and potential cap space as well. No one knows for sure since actual numbers are hidden, but there is a reason they keep going to the Red Bull well: it's cheaper if Daddy Dieter already has you on the payroll.
Cheaper for RB, but not for RBNY. Pretty sure the league is strict about those types of things. Man City couldn't ship someone making $100K a week to NYC and only charge them $1 a week on the loan. But you could see how RB Global would allow RB to go "over budget" if it was left pocket/right pocket stuff.