When you want to select a section in a long document or webpage without dragging the mouse and waiting for animations you hold click from where you want the start point to be and page up/down.

When you are trying to select multiple icons from a file browser using your keyboard, shift + arrows gives you item by item, shift + page up/down gives you pages of them.

When you are in a long document or webpage and are trying to scan the text for something and use your mouse to do something on the page, page up/down is often faster than the scroll bar and your mouse if free for pointing and selecting.

Page up/down works as previous/next in many media applications.

When you write text, see that you made a mistake in the middle of the sentence, correct it and then hit home or end to jump to the beginning/end of the sentence in one action.

When you want to select text pressing shift + left/right selects letter by letter, shift + ctrl + left/right selects a word, then shift + home/end selects the line.

In a browser home/end will bring you to the beginning/end of a page. Especially useful for long pages. In a text editor it does the same by adding ctrl to the mix.

Games and specialized software like 3d and cad use these keys all the time for all kinds of functionality.

They may not be the most glamorous keys, but they are very useful in many situations.