i too like variations - but would not single them out necessarily for only technical purposes. they also serve composition purposes - because you can see the 'mind' of the composer at work. the brahms paganini variations, to me, were very helpful to a point. practicing thirds and sixths. but, when all was said and done - i still needed help.
my teacher here in pa was extremely helpful to something i didn't even really think about. it was more the approach to the notes themselves (massaging them) and almost treating the keyboard as a sort of window to one's mind. when he asked 'what are you feeling?' the first time - i thought it was some odd psychosis type of question. then, i realized - i had no particular feelings at the time about what i was playing. i was just playing it.
now, i realize - beyond technique there is much mind. to become in tune with one's feelings is perhaps the hardest thing about piano, to me. and, to correspond it to the composer's feelings when he wrote the piece. and, not to bash ff's or play pp's too lightly. to give adequate breadth and width. to become 'one.' yes - a sort of metaphysical idea - but yet - spiritual moreso. to identify with what you are playing.
that is why i think students should be involved in picking the pieces they play. or introduced to things the teacher believes they might like (looking at their track record).
Topic: Variation structures - the best?