Originally Posted by Alf
Or is in in parallel of tournaments and practice ?
Primarily this. The obvious scenario is when you have more fatigue than you use during the week. This can happen for practice weeks especially for particularly high-endurance players; it can also happen if you unexpectedly lose early in a tournament. One example that can be common at a certain career stage is for example losing in the quarterfinals of a Slam or one of the two-week masters events. Then you only get one match, and have to fill the rest of the week through friendlies. Trainers are better than friendlies.
What law said about training possibly being better than practice weeks in some cases is interesting to me though. That's the kind of thing I'll need to experiment with.