To me it is about humility. Trying to be perfect assumes that it is
possible to be perfect, which of course is not possible. Recognizing
that the things we make are imperfect reminds us that we aren't
perfect, either.

As others have mentioned, you could build boxes with a CNC machine
could probably be "perfect," but to me one of the reasons to make
pieces by hand is so that you can see the hand of the maker in the
item. Since the maker isn't perfect, the box isn't perfect, either.

I think what the Shakers were getting at is that if we assume that we
can make something that is perfect, then we are starting to put
ourselves in a position reserved for God. I am not familiar with it,
but the Muslim belief sounds similar.

(Of course I get just as upset about glue marks as you do.)

Mark