Brooks Jensen has written about the importance of projects, especially from the point of view of a publisher. Sandy Wilson commented on my last blognote to the effect that projects are an artificial idea largely promulgated by teachers, critics and publishers, the implication being that at the very least projects are not needed, and implying they might not even be a good idea, or perhaps 'might not be good for you'.
I think the truth is somewhere between. Some photographers seem to do nothing but projects. They don't carry a camera unless they are on a project. Others simply carry a camera when they have the time, photographing whatever appears before them that might photograph well. Others do a bit of both.
Elliot Erwitt is famous for his dog pictures, but he photographs amusing and insightful relationships all over and often without featuring dogs. Edward Weston would go on trips, but then photograph whatever happened on hand while there, from dead pelicans to nudes, pottery to friends.
It's true that Edward Weston made a project of Armco Steel, but only insofar as he visited it and got a few photographs - certainly not enough to make a real 'project' of it, not enough to submit a portfolio of the images so he could get them in Lenswork. Yet one of those images was certainly considered a milestone in his work. Did he then go around a whole whack of other industrial sites - no he didn't.
In selecting images for "Why Photographs Work", I selected an industrial image from a photographer famous for his landscapes, and he quite reasonably asked if we could select a different image that was more typical of his work In another case, a photographer had 'moved on' since doing the image I selected and didn't want to be included if it meant going with an old project. He felt obliged to support his galleries which were all showing his new and diff. work.
Some subjects are best explored thoroughly and only over a long period do the best images develop. Other times, there simply isn't enough material to make repeated visits worth while - before long you simply make variations of the shots you made last time.
Of course, you also have the problem of defining what constitutes a project - wouild you call landscape in general a project - hardly. What if you limited it to mountain photography? Or a specific mountain - but what if you then find you don't have enough great images of that one mountain, so you have to travel and build mountain photographs over many years - still a project?
No matter what topic/subject/project you come up with, you can generalize it enough so it doesn't look like a project, or narrow it so much its unlikely you will get a whole portfolio of images from it.
You'd think that photographers who shoot almost at random and with no real plan or project in mind, would eventually shoot enough images they'd get lucky and over 20+ years produce a portfolio of really interesting, yet entirely unrelated images. Interestingly, I don't think this happens very often. Typically the people who can produce a sig. number of 'random' images of great power or beauty are the same ones who also gravitate to projects.
I suspect that projects are a natural side effect of being curious and interested - why wouldn't you want to explore more situations that have given you pleasure, challenge or success in the past.
What does happen is that lots of photographers simply don't have enough depth in any one subject to make a show or please a publisher and they don't because they like to spread themselves thin, which is just fine. They do need to be aware though, that this may mean they aren't going to get published or acquire fame until late in life by which time the apparent randomness of their work coalesces into a series of subjects of enough depth.
Of course, this raises the subject of how you define success in photography and that's a topic for another day.