Heads up. Today I am using many parentheses. Disclaimer aside:
The only time I've used my GPS for training rides
(other than simply getting lost and trying (very badly) to find a way home) is to add a 'marker' on hazards.
One jab of the button (on an entry level Etrex) puts a waypoint in the map for all eternity, and (if you have proximity alarms set up) will sound a loud beep whenever you move within 100 yards of the same location in the future.
Very useful for marking slippery corners, bad surfaces, cattle grids, concealed entrances, or (in my case) a dead badger that seemed to stay put for about 3 months.
In happier times, I suppose I could use it for nice cafes, lovely views, particularly pretty trees, etc.