Why sprint? It teaches the body to accomplish MORE work in LESS time. Yes, there are the metabolic after-effects, EPOC, and such. But don’t lose sight of the need, somewhere in your training, to become fundamentally better than you were. Too often, with the emphasis on depletion, people do longer activity only to find that they are becoming less and less capable. People in my network will triumphantly report back to me that they did a few hours of walking. And I don’t think that’s a bad thing, especially when compared to NO activity. But when compared to anything else, it doesn’t represent improvement in health prospects or fitness capacity.
Of course this doesn’t mean you have to hit 2000 watts (or the 1800+ featured here: https://www.instagram.com/p/B3E6ZKqj2gj/). It just means that you should figure out a way to compared-to-self improve. That's why sprint.