I generally use lower temps than that on mine (though a different hot air station), but it can take some real significant time to remove a power package from a big plane or trace. Remember that mosfets will often be soldered down to an in-PCB heatsink pad, then of those pins you'll have half a dozen or more on each primary power connection and the trace will be wide enough to accommodate that much, and one side of it is probably going to a very high current power rail (the low voltages use mean it needs to be large high current handling), so you've got a lot of copper around it that needs heating.
The preheater is nice because even if the mosfet is on a thick trace that runs the full size of the board, you can get the whole thing up 50-75C as a starting point, which means you achieve your desired temperature at the spot of interest a lot faster. I've spent several minutes heating a power device on a big power plane before being able to move it more than once. If you have something you can use as a jig to hold your hot air gun, you can use it normally for a bit, hold it over the part but a little backed off to maintain the heat application, and then go in and touch the pads with a standard soldering iron at a normal temperature - sometimes the extra little bump you get from the hand iron can free the pad up and get it off a fair bit faster than air alone. Maybe harder with QFNs, but probably still useful and quite viable for DPAKs and such.