Two examples with the same type of subject. Most of the time, autofocus is the only way to capture a fidgety rodent, like a squirrel.
They just move too quickly to capture with manual focus unless you're a pro at manual focus. Right after the above shot the squirrel bolted.
On the other hand, I had to use manual focus here because the camera kept wanting to focus on the branches and I was looking too much time with the technique of putting the autofocus spot over what you want, half-press shutter, and recompose.
So it depends. On a landscape or adult portrait you can probably do manual. On firewords displays you almost certainly need to do manual. But for kids running around, a party, unpredictable situations, you need auto-focus.
Oh, and don't forget to switch back to auto afterwards unless you want a bunch of ruined pictures because you forgot to check your settings before a new shoot!