I rarely do it for comics because I mostly read webcomics as they are being created, but it is something I do a lot for novels.
Not so much to know if the end is good, but rather because I have no interest in stories relying too heavily on suspense or revelations. So I prefer already knowing the end and enjoying as I read, without getting distracted by the possible ends. Plus, if I get bored on the way because I already know the end, it tells me that's not the kind of book I am looking for, so I stop reading and I save time.
I think it may be because I started reading with mostly classical literature, so often I knew what was in the book before even opening it.
Also, I love rereading, and don't like when books have only one life. If they can remain fascinating after 10 rereads, for sure spoiling the end is not an issue.