The advantage is that you can build up credits on your main character faster than Nedding or playing fireworks, particularly if you need to buy something that is going to cost 5,000 or 10,000 credits.
Everytime I create a mule, after playing all the games, entering codes, visiting NPC's, selling clothes, etc., the mule has a little over 2,500 credits. This also includes credits from fireworks, so if you don't play fireworks, obviously it will be less, but it should be over 2,000.
The mule will not be able to get the 5,000 credit item, unless I were to play Ned for 2 1/2 more hours.
So assume I have at least 2,400 credits on the mule and purchase 12 spider webs. I trade them to my main character, and have the main character sell them. My main character would pocket 1,800 credits.
Considering it takes about 25 minutes to create the mule and get it up to 2,400 credits, trade the webs, and have the main character sell them for 1,800 credits, I could have 3,600 credits on my main character in less than an hour.
I would need to Ned for over 3 1/2 hours to get 3,600 credits. Using the mule-web technique, I can get 12,600 credits in the same amount of time, and I would be able to purchase the 5,000 or 10,000 credit items.
Hope this helps.