At first glance, the Butterfly Spline Bit (sometimes called a Butterfly Key Bit) seems like a very simple and limited use tool. But look closer and you can find a lot of useful applications for these splines.
The router is the one tool that will handle all the dadoing and grooving you'll do in woodworking. The dado is prime-choice joinery. It follows that hoary adage of woodworking, "Use the simplest joint that will work."