I worked a lot with fansubbing when I was younger, for free and just for fans, and that was what cemented my career as a translator. It is just like you said, we do it as a hobby in our free time, but at the same time we gain valuable techniques and knowledge that will be useful for us in the future. Nowadays I don't do fansub anymore as this is now my job, but without it I would not be where I am now.
Besides that, VN fan translations are basically what made the market exist nowadays. When I began reading them, we only had a handful of good translated vns, fuwa still had torrents and the same teams worked in different projects for free. Now, if you check VNDB for official releases' staffs, you'll see many of those translators now being paid for their work at the same time we keep getting quality VNs WAY faster than before.
Also, charging for fan translations is shady as hell.