Sanding will make a wooden surface smoother, which is not a good thing
to do if you want to glue it.  Making it rougher does actually expend
the contacting surface, making the glued connection even stronger.

The best surfaces for gluing (and finishing) are hand planed faces,
with sanded coming next, and crushed surfaces such as might come off
a hot roller the worst.  What is important is not the gross smoothness
of the surface, but its microscopic smoothness.

Surfaces that are rough to the touch make poor joints because the bumps
hold the faces apart, increasing the amount of glue-to glue joining
(relatively weak in most woodworking glues, strong in something like
epoxy) relative to the glue to wood bonding.

Cleanly cut surfaces, such as you get with a sharp hand plane, have the
wood cells split cleanly open and have a vastly increased microscopic
surface area for bonding compared to sanded surfaces where the fibers
have been bent and bashed and have filled many of the spaces, which is
in turn much better than a crushed surface where the microscopic
surface are per square centimeter is minimized.

You can see some of this with a hand lens; scanning electron
micrographs are much better.  I strongly recommend Bruce Hoadley's book
"Understanding Wood" for anyone interested a readable treatise on all
aspects of wood as a material.  I found the thesis (in French) in the
following link that has some pretty good electron micrographs related
to applying finishes to wood, but the issues and problems are exactly
the same: http://www.theses.ulaval.ca/2006/23336/ch03.html

Search for images using "wood micrograph" as a search term and you'll
find many pictures that illustrate the structure of wood.  The tubes
discussed earlier probably refer to the xylem and phloem that are
responsible for transmitting water and nutrients in the tree.  In
ring-porous woods like oak they are large and obvious, less so in a
wood like maple, but clearly visible with a hand lens in either case.

Cheers,
Roger Wiegand
Wayland, Massachusetts, USA
www.carouselorgan.com