Again Hi
At first , about the reason of that resistor :
As we know , when the GS junction capacitor charged , the impedance of gate is infinite ( approx) , thus it can't become discharge ! and it will be dangerous . that resistor can discharge it . and it's value is depends on the value of Ciss and your time .
and you can test this behavior . take a mosfet , and give a voltage across it's GS junction and then , you'll see , it's DS is turn on , and if you remove that voltage , you'll see still DS is on ! but if you touch it , it will turn off ! test it , please , it will be lovely effect that you can use it on your projects!
And about saturation , i think this name is wrong ( in opposition to the BJT ) , you can say , it is on , simply !
And the reason of +15 volt for usual mosfets or +5 volts for logic level mosfets :
As we know , if you , give the voltage more than threshold voltage , your mosfet will start conduction . but not totally ! it means if you don't get it +15 for usual mosfets or +5 for logic level mosfets , your mosfet will be at linear region , and it means dissipation ! and heating ! and i don't think , that you like it !
Good luck
Goldsmith