Control inrush current
Surge is reduced by using a constant current source to charge the Gate capacitance of the P-FET. 
You can alter the amount of time taken in the linear region by altering the CC drive. 
If you drive the P-FET with a fast drive (high current therefore fast voltage change) you would get surge current.

No you can't turn the P-Fet around (swap Source and Drain) since you'd have simply a (body) diode which would conduct and drop the source voltage.