1. If you need the contact resistance of thermal switch is less than 15 mOhm, you need gold contacts. The resistance of gold contacts is much smaller than silver contacts.
2. If the rated current of thermal switch is 250 mA or less, Gold contacts are necessary. It can prevent oxidation of the contacts at low current rating. Oxidation forms an insulation film on the silver contacts and low current does not arc enough to burn off the oxidation.