I think "opportunities for career growth" is very similar to how graphic designers or musicians are always pitched that they should do work for someone for low/

I think "opportunities for career growth" is very similar to how graphic designers or musicians are always pitched that they should do work for someone for low/no pay because it will get them "more exposure". There is the possibility that the growth opportunity will end up being a good investment with regard to future financial returns, but I think the most likely case is that it will just end up with the person doing the work losing out on money.