Caleb has been doing scales in band. For anyone who has never done band or music, scales are the DO-RE-ME-FA-SO-LA-TE-DO things, like in Sound of Music. You can play a scale in any key, and learning scales helps you learn the notes in that key (which helps when you play music in that key). It's not the most interesting thing to do, but scales are important building blocks for learning music. As I explained to Caleb, scales are dead easy to play, but because they're so simple, any missteps, timing issues or other problems stick out like a sore thumb. So while they're easy to play, they are hard to play perfectly. Learning to play scales perfectly, while not very exciting, will translate into other parts of playing music and will pay off for the music student in many other ways.
I realized that stockinette stitch is kind of like playing scales in knitting. It's an easy stitch to do - one of the first we learn, but if it's not knit perfectly, our mistakes are very obvious. Other, more complicated stitch patterns can camouflage tension issues, whether they're overall tension issues, or just tension differences between the knit and purl sides. With ribbing, cables, or lace you notice the pattern, not the fine details of the knitted fabric. Even garter stitch brings a texture to knitting. Stockinette, as it should be, is smooth, unbroken, and perfect. If it's not, it really sticks out.
Of course using handspun yarn, I'm really noticing all the inconsistencies there too. No pressure!