because i found myself typing "hacker news" into google instead of typing all 20 characters into the url bar.
I'm here so much I just type ne <tab> <enter> and I'm here.
There's something unsettling about using a domain that forwards to the site you actually want to use; especially when they aren't owned by the same person / organization.
That's a long shot, but if enough people would start using a non-related domain as an alternative front door to a site that would be a very nasty way to catch a bunch of people unaware.
Even more sneaky to forward them to the real site after they log in (or to proxy the requests).