They're inherently convenient and fast to use. As I'm typing, it's far more ergonomic to take my middle finger and swipe on the screen to scroll than to reposition my hand to have two fingers on the trackpad. For extended reading I can keep one hand at the edge of the screen to scroll with a thumb instead of cramping my wrist. Zooming is quickly and intuitively obvious rather than having to figure out where my mouse cursor is, reposition it, do a pinch gesture or Ctrl plus (where IS the plus key? It seems to vary on every laptop I own), etc.
The hand repositioning was the rationale for the ThinkPad TrackPoint (which I still use daily). Right on the home row, and with 3 buttons for left/right/scroll+zoom purposes. Sadly it never caught on, possibly because the precision is not as good as a touchpad. It's even been removed from some of the latest ThinkPads.
