I’ve been watching through some of @harry_durgin’s videos on YouTube (which are fantastic!). I noticed that Harry exports between particular edits but I can’t quite grasp why there is a need for that. Could someone help explain?
I vaguely recall an explanation in one of his videos but can’t remember which one 
It’s to save processing time. All the edits are now baked into the exported file, so don’t have to be reprocessed over and over again.
Ah! Thank you, that makes sense!
I guess a second reason could be that this is a (the only?) possibility to overcome the fix pipeline order of darktable. But I don’t remember if that was @harry_durgin’s intention.
In some cases he does it for that reason too.
I do remember that explanation of him 1) avoid any potential deviations caused by effects added over each other 2) just flatten the picture at some point when the history stack becomes long - to optimize further processing speed