either because it is actually rare . . .
or because it is customized . . . .
or because it is in such bad shape no one else would have one . . .
I don't have a customized watch, unless you count "marriages".
Here's a rare one, a 1983 Phase de Lune Or, 466 unnumbered pieces

Here's a badly done redial (the distortion of Swiss Made really looks like that)
This one was bought from a guy that Bill Sohne and I have noted, "everybody buys ONE watch from ____ -- but only one."
I have more bad redials, but that's is the one that makes me cringe the most.
So, let's see them
crap
custom
or
(un-)common