Its absolutely relevant. Of course its not "legal", but recruiters like to see that you are a busy person contributing to uni life. It shows you aren't lazy, indicates you might be someone the partners would like to work with and demonstrates generic transferable skills such as empathy, dedication and social skills.
Extra-curriculars like that aren't a deal-maker and you won't get anywhere if you don't have good enough academics, but in general law firms do expect people to have a range of extra-curriculars on their CV. I think volunteering in a care home is quite a good one.