something unclear with sed for me

[Andras Lorincz <andras.lorincz@gmail.com>]

I want to replaces all multiple spaces with one space. My first
attempt was this:

sed -e 's/\ */\ /g'

This replaced all multiple spaces with one but as a side effect, sed
inserted a space between all characters. Playing a little I tried
this:

sed -e 's/[\ ]\ */\ /g'

and this works. The fact is that I don't understand why the first one
doesn't work. Can someone explain me that?