I'm sure this must have been asked before but I'm finding myself incapable of phrasing the search query correctly to find it.
Guitar and bass (and more generally all 'necked' instruments) necks are usually (always?) between 1 to 3 mm thinner at the nut than they are at 12th fret. Are necks carved in this manner for playability or construction/structural reasons?
In a very large neck, like 6 string bass or 8 string guitar, a 2 mm difference of thickness, nut to 12th fret, wouldn't be too noticeable compared to the bulk of the neck volume in your hand. Moreover, the neck width and therefore its volume decreases anyway moving up to the nut. So what justifies the extra effort to have this slight slope at the back of the neck?
By the way, what is the name of this difference in thickness? Searching for "slope", "angle", "thickness" etc. yielded results about neck to body angle, or topics of "fast/shredder necks" instead.