I use the 45-point AF with distant, not so well defined, subjects. For example, but not limited to, distant foliage.
I always check if my subject is in focus and do not hesitate to fine-tune with manual focus.
As Paul mentionned, I use the 45-point AF with moving targets.
On other occasions, I use the AF system with one point (either central or lateral).
An example for one-point AF is with wildlife. In this case I like to focus on the eye of the animal (if possible). The central point is the most accurate.
Don't forget that in some AF modes the exposure is linked to the selected point (or group of points).
Hope this helps,
Francois