It's about being able to rebind the jump button so it's separate from "move up".
Quoting a post of mine:
Vote here: http://goo.gl/mod/VAhBI have a suggestion: how about introducing an alternate control mode where there is an additional jump button and "up" (e.g. "W") is only used for moving up ladders etc.?
Generally speaking I dislike games with "up to jump" controls and have ranted about that when it comes to many indie titles (they seem to get it wrong more often than not) since logically speaking "jump" is an action, not a movement direction.
On a much more practical note "up to jump" is just not very ergonomic and strains the hand - at least for me.
I rebound "up" to space but that makes climbing stuff counter-intuitive. I'm not saying the current control scheme should be changed for those that embrace it - just an alternate one introduced for people like me. ;-)