It makes sense from a marketing standpoint, IMO.
If the phone is as good as it's supposed to be, they've given Sprint a reason to promote it for them. Sprint gets an angle to look forward thinking/innovative and probably pick up some new customers. Sprint already heavily advertises and now this phone will be featured as part of that advertising. Sprint needs ways to differentiate itself in the marketplace and this could be a solid way to do it.
For Essential, it's a way to create demand an a bit of an exclusivity feel and get a truckload of free advertising on the back of a giant. Essential isn't Apple or Google. They won't have the out of the gate manufacturing or marketing channels available that either of those two giants have so this will also provide a means of somewhat controlling demand.
For people using Sprint, they get to feel a little smug if they whip this phone out and it IS awesome. If it's as good as it could be, you create a means of having that drool-inducing effect that makes people seriously consider switching to get one.
I'll go ahead and tell you...I want one. I think it looks like a great design, but I'm on an iPhone right now along with my entire family. The parental control features basically lock me into an iPhone until the kids get older. It's a small network effect....but whether essential was on AT&T, Verizon or Sprint...I still couldn't get one because I'm stuck on Apple and not AT&T.