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There are many reasons for this, OP.
In Nerissa case, she loves singing, and will do it for fun, if she gots money enough for other sources. So yeah, imagine that people would put time and effort on things they love, even when its a "waste of money".
For others, it can be a portfolio. The same way many people build their portfolios, putting a lot of effort, to convince people to hire them. If a talent has songs, performances, voice acting, with a meaningful enough viewership and approval, it can be used to advertise that they can be hired to do it.
And many people fail to realize that "low views' from a holo are still very solid views for anything else.