Besides being a mouse, you can map it to a virtual menu with as many shortcuts as you want, the shortcuts being key presses/combos, allowing you to bind way more things than would otherwise fit on a controller. You can also set up mouse regions so that the trackpad only maps to a part of the screen and will instantly jump there, like for moving around an inventory window or minimap.

For games without controller support or where KB+M has an advantage, the trackpads are a game changer. Imagine a hotbar like in Minecraft or Terraria, but instead of having to go left or right one at a time with your bumpers, you could spawn a radial menu with the touchpad and instantly flick to "8" or "5" or "1" to select those slots.