I'm trying to think of a single actually useful thing I ever worked on..
You have worked on earning money to live in your world. Good job. Don't over think work.
You're writing the code, or producing the documentation, managing the project, performing QA, or whatever else your role is, and in exchange your company is paying you money. That's the bottom line. If you think it's a useless project, then you should be even doubly grateful that a company exists that will pay you your (presumably good) salary to create something useless! I worked on a totally useless project in the past, a lot like some of the comments here describe, and I went into work every day thanking the stars that my company was stupid enough to pay me to do this!