If you speak to the suppliers who spent the time when the RC390 racing scene was a thing (does it still exist?) such as grey area KTM then they will tell you the stock injection has enough headroom for the modest airflow changes that most riders make. Naturally a point comes when one is needed but is not always a factor in deciding a piggyback.Piggyback ECU's can't overcome phisical lack of fuel. Engine modifications on intake and exhaust will require higher flow injector or higher rated fuel pressure regulator to avoid lean conditions at WOT or open loop... Higher rated FPR can be used with the stock ECU also.
Plug in piggy back ECU, Why one?
As for why one, well this is my opinion but a locked ECU on the gen 2 means not so much point putting it on a dyno for lots of runs with that slow not very self learning process and emissions controls built into the stock map.