I think you can include both and ideally should include both.
In general, when wring an answer for an extracurriculars application form question, you always want to link the experience to a skill that is relevant to the role of a trainee solicitor. Nonetheless, given the particular language chosen by the firm when formulating their question ("how others have benefited from your involvement"), I would interpret this as a clear suggestion that you can discuss the relevance of an experience in a wider and potentially more communitarian context - i.e. you can explain how you developed skills/interests/qualities that were good for you and other people more generally, rather than only focusing on skills you developed that benefited you in the narrow sense of being good for your career. Presumably, the firm chose this drafting because they consider this broader personal development to make for a better candidate to recruit.
However, as I said in the beginning, I would try to discuss both (i) this wider personal development and (ii) the development of more specialised skills relevant for a trainee. This is again as a result of closely following the logic of the language: "what you feel you have gained from them" seems to me to suggest this second and more you-focused perspective, and using "and" instead of "or" suggests that the firm would prefer it if you could write about both rather than only focusing on one.