What am I clearing pressing that button? It’s just one of those, it will fix it, who am I to question successful results.
I’ve been doing it since I first used Blender but never questioned it, just glad the weird blotches on my mesh are gone.
Some models to have custom normals because they are used in some way in a game engine (mostly some lighting “trick” to make the modle look better)… most of the time just deleting them and making blender do the work (for example smooth normals by angle or bevel modifier) just does the right thing… (or not… and you have to add some sharp edges… or creases… or… 
).
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That makes sense. Thanks for info.
Now I’m curious to see how custom normals are made.
You may have a look here: