I was asked about my photo ref from my previous post. As you can see the figure in my final is fairly different. But the ref allows me to flesh out the piece and add in details and information that I think improve the piece greatly. I could try and make it all up from my head but even then I'm just referencing the vague images in my memory of things I've actually seen. Even if you have photographic memory I'm doubtful you would have seen the exact forms you need with the exact garments, color, lighting etc.
The trick for me is to maintain the stylization and freshness in my sketch and wrap the information provided by the photos around it. Sometimes my sketches are so far off in anatomy I make some minor changes. I always sketch, design, compose out of my head. This way I'm not bound to anything that may limit my design. When major issues are solved and I feel the piece is ready. I then take my ref trying to match up the model to the figure in my sketches. I hear from a lot of aspiring artists that they are looking online for the "perfect" ref. I feel this is very limiting artistically. And to be quite honest if you spent the time setting up a photo shoot(who doesn't own a digital camera nowadays?) you would save yourself some time and have ref that is exactly what you need. The more I take ref, the more I study it and try to understand it, ie folds, drapery etc. the more I can file away into my visual vocab. Over time I become more familiar with the subjects and when I do need to make some of it up out of my head I can do so with a little more confidence.
Another quick point before I forget. I think when you are constantly making things up out of your head they tend to repeat themselves. The same folds, the same hand at the same angle. You get the picture. A lot of cool unexpected things can happen in your photo ref. This too helps to keep your work fresh.
Here's another example of some of my previous photo ref.