The idea is that not all goal scorers are necessarily going to fit the types of chances a team creates, or what a manager wants a striker to do. The example I've used before is Ciro Immobile: He was not a good fit for Dortmund at all, even though both before and after his time there he was and is extremely productive both in terms of slotting and creating chances for others. But how they set up didn't work with how he plays.
So if Kean goes out and gets 25 this year, management has to decide if foregoing the massive amount of cash they can get for him is worth keeping him around as a substitute, or if they actually expect him to be challenging DCL for the starting xi. That cash could probably buy two younger players who fit the currently much more coherent style of the team. They wouldn't be guys who you'd expect to score 20 in a season, but if you don't expect that Kean would score 20 in this team with the other striker starting, he wouldn't either, and you are maximizing your return.
Also it would probably help with some FFP accounting stuff in a pretty large way.
I like Kean and hope he comes back to Everton and is successful, but if he gets a big return and the club can get a giant return out of that, given DCL continues to score for fun, selling would make sense.