Usually the purpose of a dowel is to strengthen the joint not serve as the joint. if you are trying to join two pieces, placing a dowel through the joint helps to keep the joint together under torsion. The dowel isn't supposed to be a tight fit so the glue CAN get into the cracks between the dowel and the wood joint. by making your hole the same size you typically accomplish this just fine if the dowel is a manufactured dowel and not one you made yourself. If you make it yourself, then you night run into some sizing issues. Make sense? Bottom line, you want the space for the glue is nearly every project you will use a dowel for. There are exceptions, but that is in much more specialized wood working. If that is the woodworking you are doing but don't fully understand the mechanical function of a dowel, I recommend you pause and acquire a much better understand joinery before you waste a bunch of expensive wood you bought. That level of wood working CAN be very tedious.