This part is really only typical of non-fiction books. It shows that the book is well organized, clear, and informative. (In fiction, each chapter isn’t really a standalone piece of information and are divided more for dramatic effect that anything else.) It provides the agent/publisher with a better feel for what exactly is in the book.
The summaries should ideally be only a paragraph, showing that you can be clear and concise is always an encouraging sign. If you ramble in your summary, what will you do with even more space?
A table of contents is particularly great if your chapter titles are clever because it highlights this aspect of your writing quickly. It says “look I’m funny and I know what I’m talking about!” If the chapter summaries read as incredibly dull and dense, how much duller and denser will a full book be? Make sure you get your point across without overburdening the paragraph with information. Remember, this is not the actual book. You don’t have to explain all your concepts. You are just giving a taste of what’s to come so that they will want to find out more about the topics and ideas that you discuss.
And try to make the information interesting!
What Did You Learn? 