[old_tv_nut]

In order to work properly, the degaussing coil must start with a large AC current, but then it must decrease gradually over several cycles before reaching a vanishing level. This will have the desired effect of recording the opposite of the Earth's field into the shielding and shadow mask, thus providing a (nearly) field-free space in the CRT.

So, that's why I think the capacitor and diode are doing that somehow. If the current were still full strength when suddenly shut off, it might strongly magnetize the shield and shadow mask to some field that bears no relation to the Earth's field.

I think the thermal delay waits until the AC current is low enough that it can no longer magnetize the shield (due to the material's coercivity), although it is still strong enough to cause a purity disturbance, and then turns off the current completely. The usual thermistor circuit reaches a level of current low enough to no longer disturb the purity, without using a switch.