Recently I heard someone speak on the topic. I believe it has something to do with that after you construct the block you have a slight advantage in speed (actually fractions of a second) and you can “propose” a new block (this time empty) and then if you happen to find it within a few seconds you get additional block reward (no fees though).

I’ll try to remember where did I see the videos. Something tells me it vas Jan Capek from Braiins on What Bitcoin Did podcast a few months back.