I'm doing it really for 3 reasons:-
1) To learn more about the work itself.
2) To get paid while I learn.
3) Another income stream would help solidify my small business, as I become even less dependant on my main clients and private clients.
I think I know enough as it is to offer a good service for the prices I charge (I'm sure you'll agree) and I'm focusing on making websites that are targeted to bring in customers SEO, online forms etc. So I'm sure that I'll get some people / businesses who will think its a very good deal.
really not a good idea to learn as your building someones web site that they are paying you for. If you do somthing that bad that trips up, they have you over a barrel and can sue quite easily.