If you, the model, are in the process of building a portfolio, working for time will enable you to have work from different photographers in your portfolio. This is important to demonstrate your versatility as a model. Nobody wants a model who can only master one facial expression and one pose. It can also save you money.
Prints for time seems to work best for (aspiring) models who are committed to their work and who are able to express a photographer's and their own ideas. No photographer in the world can produce good results if you think that modelling only involves standing around and having your photograph taken.
I do PFT and TFCD for personal projects, to realise artistic work, to experiment with new ideas, equipment and materials, and - if you are very, very special - sometimes just for the fun of it.
It is important for you to realise that the photographer, too, wants to get something out of a prints for time shoot. The purpose of a prints for time shoot is not just to provide you with free photographs for your portfolio. The photographer will expect you to take the shoot seriously. Give it your best.
Of course, you will get free photographs that you can use in your portfolio. I also usually do some shots that I know to work well in a model's portfolio, no matter whatever other ideas I may be working on.