If you want to succeed as an illustrator, you will need to have an effective illustration portfolio. Clients will assess your skill level and decide if they want to work with you based on what you have to show in your illustration portfolio, and, for this reason, it is important to give a good deal of thought to what you include in it.
A portfolio is always a work in progress. You will always need to keep adding, removing, and improving to stay relevant in a changing market. A thing to remember though is that you cannot be all things to all people, and neither can your portfolio. You will need to streamline your portfolio according to the market or markets you want to work in.