If you are unpublished or have just a little publishing experience it is actually harder to find an agent than it is to get published! You do not need an agent when you are getting started in the field. As your career develops you may consider looking for an agent but many illustrators manage their careers on their own.
Once you have some experience then you will need to decide if the 25 - 30% that an agent takes from your earnings is worth it. Can an agent bring in enough more work, or negotiate better pay to justify it? You are the only one that can make that decision for yourself.