For me it's that 85% of the time I use networking, you're using something that I learned by myself. But that 85% accounts for only 10-15% of the knowledge. So there are a lot of concepts that I learned in school (like DNS, link/physical layer, microarchitecture, etc) that I rarely use. But just because I don't use a lot of the stuff often doesn't mean I never use it. And even the things that I really never use (like programming an FPGA) aren't things that I regret learning.