There are many good reasons besides the fact that people might be interested in these. One of them is "Search Engines"! There are search engines that specifically look for such pages, and publish only these pages to their database. If you don't have such a page, they just move on to another site.
It's important to create a FAQ's (frequently asked questions) page, because at times, people do type in questions into search engines, and find the site that comes closest to their search terms. Let's say you have a Services page, and in it you say, "We can help you save time." Now, no one really types in "Help you save time," they type "Help me save time." So you can have this question phrased correctly on the FAQ's page and then link to the Service page that talks about this in more detail.
Note: It's true that search engines can be smart about such inquiries, nevertheless, you would have a higher chance of being found for the exact phrases.
About Us: This page is read by most robots (search engines) and is listed first in the category which defines your goals, purpose and your company's mission & vision. Have you heard about Wikipedia? This site and others similar to it are a few examples that read your "About" page. The more places you are listed, the more important your site seems to be, and thus you are ranked higher and found more rapidly.
SiteMap: The most important page you may ever need! If you don't have a central place that contains a link to all your pages, you could be just not interested in being found, or you are simply unaware of the fact that, let's say, Google, loves you for your SiteMap, not your money. So let's call Google a unique type of gold digger, or better yet, site digger. You list them, and they will come.