I worked for free to build up skills in a job area I wanted to work. It helped me learn and practice the new skills and I put it on my resume. It helped me to land paid freelancing work with another company, so it did benefit me.

If you are gaining new skills and experience, it seems valuable. My concern would be that if you are seen as a volunteer and 'unpaid', it might be difficult to get paid work from those clients that know you. You would have to find new clients.