Ian Frazier, writing about his experience developing a tool to remove plastic bags from trees:

From that point, in a small but real way, my life changed. Having the exact right tool for a particular job is always satisfying, but when the tool (and, indeed, the job) never existed before, the satisfaction is multiplied. Plus, what we were doing, in addition to being fun, actually was benecial to society. In an over-full urban environment, we had found our niche, one we had all to ourselves. Nobody else in New York City, or in the world, was taking plastic bags out of trees.

And later:

Now I understood, a bit, how people felt who had worked on the construction of some major public landmark like the Empire State Building.

The fun part about developing a useful tool is that it doesn't really matter what it's used for. The fact that it is used, and that it was never used before, is plenty.