An excerpt represents not just a quote but a complete passage from a book. It can be a few lines taken from a poem or a novel.
Mostly excerpts are used for complete understanding of a piece of writing. It is easier to critically evaluate a piece of writing when it is broken down into excerpts and then expanded on. It enhances the understanding of the reader.
For example teachers might give a single paragraph to their students and ask them to expand on them and judge their understanding through it.
