...really do the agency work? I do realize that a good portfolio is what counts most. But I'm asking because 350$ sounds a lot if they actually don't try to help you to get jobs.
Well, sites like Children's Illustrators, the ispot, etc.. they don't do agency work because they are not agents. Those sites offer exposure. You pay to to be on their site and show off your portfolio.
You of course have to have your contact information there and should have a link to your personal website with more work as well.
Art directors and editors visit sites like that looking for talent. Sometimes they are looking for an artist with a style in particular since they already have the manuscript in hand. They can easily find them on places like these because it's quick, they can do advance searches regarding mediums, styles, etc.
Some sites offer that your work be on a book too. They send that book to publishing houses. Others go to shows like Bologna Book Fair, Frankfurt etc.. and promote artists work there. I know CI displays a big screen where they show images of artists' work.
Yes, online portfolio sites tend to be pricey. CI is one of the least pricey actually. I know it may seem like a lot of money but I always think of promotion as a must. Specially if you live far away from publishing houses.
Every artists is different. I am not saying everyone should advertise in places like that. Some artists just send out promo packages and postcards regularly. Other like to fly to NY and do portfolio drop offs and meet the people face to face. Advertising online helps in my opinion. If you can do all those things well great. But almost no one can so you have to pick some way to advertise and go for it.