The first is that somebody else will eventually have to work with it, decipher what the heck I was trying to achieve, and generally get tripped up and slowed down by it.

The second is that eventually I may have to work with it again, and sit there scratching my head and wondering what I'd been smoking when I wrote instead of focussing on the changes I need to make.

The point is to code for maximum clarity to humans whilst still achieving the desired functional and non-functional outcomes on the machine. You will use varying terseness as part of doing this because terseness isn't the point: it's just one of many means to your end.