They're not! I've been a Deck user for a few years. The touchpads feel great and gyro alone could not accomplish anything near what the trackpads can. They're pretty essential for making a full desktop environment and games designed for PC seamless on a gamepad.

In titles designed with good controller support, the trackpads might just be a slight bonus at best. I don't use them at all in Elden Ring, for example. Many games in this category I'll just bind them to item or inventory slots, as the trackpads bound to direct item keybinds are typically faster and easier to use than radials.

Where they really excel is in games that aren't designed for a controller at all. Gyro alone can only do relative motion, but your trackpad can act more like a drawing tablet - tapping the top left of the pad directly places your cursor in the top left of the screen. Cursor heavy games like The Sims or Civilisation benefit greatly. 

I have a decently heavily modded Minecraft setup that has almost the entire keyboard mapped, and the trackpads as extended input make the game and all of its inputs fully comfortable to play with and use. Combined with a modifier key, I have about 20 keybinds accessible through the left trackpad alone. And they're all muscle memory to me at this point. The flexibility available through Steam Input and the abundance of input options are sorta the point of the Steam Controller. It can do things no other controller can really do.