I think working for free, or collaborating, is questionable when one member of the team stands to benefit financially, or just doesn't want to spend money. If it is a collaboration then everyone concerned should benefit to the same extent.
A few of the justifications used for uneven collaborations are;
They can give you experience and knowledge you wouldn't have access to any other way. This one could be a good reason, depending on who is making the offer.
It might lead to paid work. You will have to weigh up how likely this is to pay off. If I were to work for this reason again it would take a lot of persuasion. When I did, it quickly became apparent I wasn't valued or appreciated. I should have heeded the warning signs when they arrived over an hour late for the first shoot. When they did it for the second time I called it a day. I hoped it might mean a network of people needing my photographic services. With hindsight I realise this was never going to happen.
And the final carrot used when asking someone to work for free is exposure. This is a real red flag. Who will be exposed to your work? Are the thousands of Instagram followers likely to be in need of your services? Don't forget there are ways to buy followers. The number of these potential customers on any social media platform should be viewed with suspicion.
Working for free can affect your reputation. The quality of your work might not be what people know you for.
I follow photography forums and Facebook groups. The collaboration ones are changing into places to post requests for people to work commercially for free. It's not just photographers, people want models, MUA's and studio time without paying. How do these people and companies expect others to make a living?