Probably been discussed a lot but I thought I'd carry the discussion over from Ortega's recent thread which received quite a lot of advice for him to use one and some saying that they are - how to put this mildly? - not really necessary.
Anyway, I use them regularly, I think of them partly as the equivalent of a stop-watch for athletes, or a measuring stick for the high jump, or [please continue with your very own analogies...] but also, of course, a basic system to make you really conscious of beats, metre, subdivisions etc.
When I had a space, teaching in schools, I used to challenge pupils who said they couldn't "feel" the beat to walk from one wall to the other UN-rhythmically. It is very difficult to do.
So yes, we all have an innate sense of pulse. But playing a complex musical instrument adds many layers of difficulty between what is natural in us all and the concentration required by musicians to be 100% accurate. And yet, ironically, the slower you practise, in order to be able to hear, feel and achieve everything, the more difficult it becomes to stay focused on the pulse.
Used intelligently the metronome helps you to know your current speed limits, and work to extend them, but also to devise methods by which you can practice smaller, difficult sections using sub-divisions of the pulse as your metronome speed as preliminary targets.
For myself, I generally find that the shift from eventually being able to play something very slowly - but totally comfortably and accurately, however long this may take - to the correct, faster tempo, is almost instantaneous. Yes, I could now do that without a metronome but I still like the external discipline that it offers.