You don't always have WD-40 when you need it, and not all jobs require the power of the full spray. For portability and more accurate application, you can use this pocket WD-40 pen.
I don't use WD-40 very often, but I do have to fix squeaky hinges from time to time. The pen makes it easy to essentially "draw on" the exact amount of lubrication needed without worrying about overdoing it and causing a mess.
While not a tool you'll need every day, it's one so small, cheap, and occasionally useful that you ought to add it to your toolkit.