I have a few projects on going. I don't think they make any difference from a technical point of view, but they do provide an opportunity to focus ones ideas about a specific subject and how to present it over a range of images.
It is also satisfying to work over time, accumulate a body of work and be able to have enough good work to have a portfolio of the project. That does not mean that for me a particular project is ever finished. Sometimes one project can morph into something completely different or refine what you are trying to communicate. I was working for a couple of years on photographing old churches in the area, especially ones that have been abandoned. During this project I became aware of new groups and congregations starting up who convert many old commercial spaces to churches such as convenience stores, old theatres, warehouses and even gas stations.
The former project detours into something that for me is more interesting and more of a challenge to present.