The most obvious benefit of metronome work is, of course, that it emphasizes accurate counting and a regular beat. And more, there is truly something unique about that consistent, even aggravating, click that lends itself so well to woodshedding - it is our musical bread and butter.
But another aspect of metronome work often goes unrecognized: it helps establish an objective baseline in practice and tracking outside of lessons. It is here that the metronome really shines when it is used correctly.
Learning any piece is a complex process - especially a more advanced one. The usual approach is to work in smaller and more manageable sections, and a lot of care is often taken between teacher and student to approach the piece in an ordered way.
Still, many students are left with a vague notion of how each section is progressing, what sort of work has been done on it, and where things stand. With only one "checkup" per week, this situation leaves many practicers with a sense of uneasiness; an "I've played through things and they seem OK" feel that saps motivation and progress. This is especially true with longer pieces, where a day or two can pass without giving meticulous attention to every single bar.
As a possible solution, I often have students "clock" themselves in every section - finding a speed where they can consistently control all of the elements of their playing and repeating this with the metronome (with full concentration!) several times per section. (I should point out that I do this *after* the student has some familiarity with the notes, since introducing the metronome before that only causes confusion.)
Of course this is nothing new - but it works. Instead of being able to say only "I played these 8 bars a few times and they went tell", the student can now say "I practiced these eight bars three times at 72 to the quarter with consistent notes, shaping, dynamics, articulation, etc."
This slight shift of focus offers a powerful antidote to sloppy practice, especially in students or pieces that seem to resist a consistent polish throughout. When these eight bars are returned to after a day or so, the student now has a written record of progress rather than a foggy memory of where things left off. This allows for practice that can be gauged and monitored without the teacher's watchful eye.
Needless to say, this sort of work can easily be overdone to very unmusical effect - I would never suggest someone practice only with the metronome clicking away. Indeed, the metronome is almost always frowned upon at least a bit. Its incessant ticking is associated with playing that is all too . . .well. . . metronomic, and it conjures too easily the image of the stern scowling teacher with ruler perched inches above a hapless student's hands.
But we would be wrong to dismiss the tried-and-true technique of metronome work simply because they are sometimes overused while novel techniques seem more fresh and exciting. Try it yourself and you may be surprised!