> Part of good design is that it's discoverable.

A more fitting term here, perhaps, is "affordance" (though "discoverable" is also accurate, but "affordance" is a more technical term here).

A mug without a handle can burn your hand. Adding the handle is a way to avoid that. The handle itself is usually designed to look like a handle. It's clear what the handle does, based solely on its form. You don't need a manual or an example or a hint to use the handle - most people, intuitively, will use the handle as intended.

The handle has "affordance". Waving your hand over the top of a computer in order to use a 'button', in contrast, has little to no affordance. The comma on this site acting as a button has no affordance. And so on.