I see people do this from time to time and I've never understood it.

I do this and the biggest reason is to avoid mis-typing and hitting a phishing site.

Interesting, and fair point. I tend to type things explicitly for the opposite reason, because I trust my fingers more than some obfuscated typo in an URL - but I see logic on both sides.

OTOH normally when I ask the people I see doing this why, their answer is more along the lines of "dunno" :)

One more reason I do sometimes this because I remember the website but don't quite remember the domain (was it .com or .net or .org)

I do understand that, but the poster specifically said typing "xyzx.com", not "xyzx" - and I definitely see people doing that

Yup. I have a friend who goes to Google, types "facebook.com," then proceeds. I mean the whole facebook.com

Obligatory: http://lmgtfy.com/