Typically agents and publishers want to see your first three chapters (or somewhere between the first 10 and 50 pages). Don't include extra. If you have a prologue, it counts as the first (since the first cannot be understood without it.)
This is the opening of the story, and most consider this the most important writing in the story, beginning with the first line as the most important LINE of the story.
This is because, if readers are going to put the work down, this is likely when they put it down. The first ten pages is when they have the least possible commitment to the story. Agents want to be grabbed right then, preferably immersed from the opening line, and they want those first ten pages to be engaging enough that the reader will be dragged through them, and that should be enough "time investment" and words (2500 in ten pages) to get the reader interested in what happens next, or to get them interested in your character and want to see what she does next.
Another reason to judge first pages and chapters is that later chapters can (rightly) depend upon context and character relationships and incidents developed or described in earlier chapters. Read cold, they can fall flat, even if they would be fine or great knowing what had gone before. That won't be true (in non-sequels) for the first pages or chapters. There, the writer must treat the reader as not knowing anything about the setting or characters or what is possible in this world.
That is difficult writing to do! The writer has bucket loads of information to dump on the reader, and how they do it, how they start a story cold, is the best measure of their skill.
I should think contest reviewers or grant reviewers would be interested in the same thing, as a test of skill. That is how agents and publisher's professional readers see it too. If they want to keep reading after ten pages, or thirty, or fifty -- If they haven't been irritated enough to quit -- That is an author that can begin a story and engage an audience. They request the full manuscript to see if you are also an author that can finish a story without disappointing the reader.