Dowels are used to strengthen joints and support shelves. Store bought dowels however aren’t that great. The machines that make them don’t take the grain of the wood into account. So they aren’t as strong as dowels that you make yourself. Also making your own dowels is cheaper.
In the first video we have a traditional dowel jig. That was passed on from father to son. I the second video he creates a shop made dowel plate. In the last video he uses a store-bought dowel plate. For which he has made a jig.