Why snubber instead of simple diode?

[KwangHeon]

I'm studing the flyback smps circuit.
As far as I know, the primary side snubber circuit is required for the protection of switching MOSFET.
Calculation of the values of capacitor and resistor is headache for me.

How about using a diode (like 1N4005 or 1N4006 just below the break down voltage of MOSFET) between the drain and source of MOSFET instead of snubber?
It does not require any calculations.

But I haven't seen any schematic of this kind.
What's the problem with this diode only simple circuit?

 

[FoxyRick]

The snubber's function goes beyond simply protecting the switch from back EMF like when turning off a relay; SMPS switches run quickly enough for other things to be an issue as well. The snubber can be designed to reduce ringing, prevent spurious firing of other switches due to spikes, and generally increase reliability of the switch.

See TI's document here:

**broken link removed**