When using dowels to join wood you want a tight fit in the hole. But if you’ve ever tried to pound a dowel in with glue in the hole, I’m sure you’ve found that the glue creates an air tight seal around the dowel which doesn’t allow the air to escape the hole, thus stopping the dowel from going all the way in. To fix this problem I cut a small slot lengthwise in the dowel.
Factory made short dowels have a spiral cut or multiple flutes around the dowel which spreads the glue more evenly around the surfaces, which does make a stronger joint.
This jig makes the slitting process go quickly. Since every time you buy a dowel, the diameter is always a little bit different, sometimes you can never find a drill bit the right size for your dowel. When this happens, drill a hole smaller than the dowel, then cut a deeper slot in the dowel. Deep enough for the dowel to fold over a little, in order for it to be forced into the hole.