What is the purpose of clamping the gate and source of a mosfet
Like everything else in life the answer is:   It depends

Depends on the nature of the circuit (does it have highly inductive switching loads?  Is it 'life-critical', or is it a McDonalds happy meal toy?  Or is it the fuel regulation control for a 737 jet??)  etc..

MOSFETs work just fine without them

My company makes consumer products by the 100's of thousands.  We use millions of 2-cent MOSFETs.  Everything is battery operated.  There's no known significant issues we've ever had with MOSFETs failing.  We never use those protections  

If I were making electric cars, I'd have these protections on my motor controls. 

So...   It depends