The scenario you give at the end is different. This isn't necessarily about finding advertised unpaid work - it's about getting your foot in the door and can involve offering to work highly discounted, or for free, in exchange for mentorship etc. Once you have the skills/connections from this initial step, you can advance your career. It's about taking calculated risks to advance your own interests - that otherwise probably won't get advanced as far/as quickly if you didn't take the risk(s). It's certainly not about working like a dog for someone trying to advance their own interests, while disregarding yours ("we need a website that's like Uber for X for $0")