Purpose of this diode
If you look at the data sheet documentation for the LT4256, it also uses an IRF530 but, in that circuit, it relies on the internal bulk diode for any necessary reverse protection: -

I expect that the person designing this circuit may have been unsure about the presence of the internal MOSFET bulk diode and may just have played safe and included an extra one.
The internal bulk diode is rated for 14 amps continuous current and has the same current rating as the MOSFET itself.