While it may seem a naive question, wouldn't a saline infusion be quickly absorbed without lasting effects?
Yes, after quite a few hours, it would.
One effect is the fact that it helps stretch the skin which allows for further expansion, in her case, fat transfers. 
That was my thought, a very temporary stressing of the skin, perhaps to ready herself for another fat transfer in the very near future. Basically doing a artificial Chelsea maneuver that disappears unless filled.