Amnesia.
Anyway, immersion and the want to adjust movement speed in such a way is completely player dependent. You seeing no reason to do it does not mean that others do not.
I've adjusted movement speed to creep in for that special stealth-kill, to bask in the awesomeness of a brilliantly crafted walking animation glistening in some fancy lighting, etc (you haven't seen cool until you've seen me walk around Firelink Shrine and position myself meticulously on steps in order to have one leg bent at the knee, mang).
It's something that I've taken for granted for so long and it is so simple, so I'm just a bit shocked. Trade-offs and opportunity costs, I guess.