A deliberate miss is "used" to avoid leaving your opponent with a clear
opening for a break. It usually happens when a player has a very difficult
safety shot and, rather than make a genuine attempt at a difficult shot that
could go wrong, decides to make no real effort to hit the ball on and leaves
the cue ball in a safeish position for his opponent.
The logic behind this is that the penalty incurred (at least 4 points) is
better than leaving your opponent with an easy chance of compiling more
points than this. The only danger is that if you have a qualified (or
competent) referee, the deliberate miss will be spotted and your opponent
can ask you to play the shot (not necessarily the same one) again from the
original place the cue ball was sitting or can ask you to play from where
the cue ball finished, if he feels it leaves you with a harder safety shot
than you had originally. So you will be at least 4 points down and still
in the same boat.
A correspondent said that it's very hard to apply the miss rule to
amateurs: that's true enough, though when a very poor attempt is made
there are few complaints in our local league if a miss is called, though
it doesn't happen much, especially where the standard is a bit lower (i.e
where there would be few breaks over 50).