Hi Jade,
It would probably be useful to play the piano if you want a career in music. For a start, having some knowledge of the piano helps massively for academic qualifications in music, for example, the composition at GCSE and the composition, harmony and stylistic imitation at A-level. Then, if you want to do music at university, you'll need to play the piano (at least grade five) to even apply for a lot of courses, and your piano skills would obviously be a great help for the work you do there. However, there a lot of people here who can tell you far more about this than I can. I've got a vague idea of your age from some of your other posts and I'd say you have plenty of time to get to that standard by the time you're old enough to apply to university.
I wouldn't recommend being self taught either - I haven't had formal piano lessons since I was little, and I doubt it's possible to be more inconsistent than I am. One day I'll be playing Rachmaninov, the next I'll be messing up things out of the grade two book. This is something I'm looking to address (I'll be looking for a piano teacher starting in June and telling them to start me from scratch), so if you want to play the piano get yourself a teacher. Don't end up like me, with increasing regrets of what I could have done with my music if I'd played the piano properly in time for my UCAS form!
Helena