For the purposes of this answer, I assume that a cached page is a page that is stored on a service like the Wayback Machine or Google’s cache service. That is different from the pages or portions of pages that are stored on your computer’s internet cache. If this is what you meant to ask, then two reasons to access cached pages are because the site you want to access is down or you want to look at an older version of the site that no longer exists “live.”
Cached pages exist on different servers and different sites than the original pages. That means that an outage or other problem that affects the original page will not necessarily affect the cached pages. If the original site is down for some reason, you might be able to access the cached pages. This allows you to, for example, do research that you need to do if your homework is due soon and the page you need is not accessible.
Cached pages can also be from a relatively long time ago. A cached page can be a snapshot of what a particular website looked like years ago. There are a variety of reasons you might want to access such a thing. You might want to see how web design has changed over the years. You might want to see what was said on a given site at a given time. For example, if you are interested in politics, you might want to look at what people were saying on a discussion board when President Obama was elected or when President Bush decided to invade Iraq. These types of information might not be accessible unless you access cached pages.