Why do they do it?
They do it because its a good deed, and if they see that another person needs the item more than they do then they help them out by sending gifts. Some do it because they love to make others happy any way they can, and knowing they put a smile on someone elses face makes their day!
Its very rare that you may come across a yo-angel in-game, most of the can be found on the forums. The ones that are in-game are usually very picky on who they gift, they usually just like to gift their friends and that's all.
Although they have been around since the start of the game, most of them began to appear when the wishlist threads began to come into place on the forums. All the gifting and generosity caused even more to come out. This soon began a new type of yoville players! The Yo-Angels were born!