Maybe you found that section particularly interesting and would like to look back at it later? Maybe you're reading the book for a class and instead of taking notes in the margin, you're taking notes on the Kindle.
I actually find myself highlighting stuff all the time. You can also see areas of the book that other people highlighted, which adds a kinda neat social aspect that isn't usually present in reading.
Did you mean to ask why is this a relevant comment? Because aaronsw mentions highlighting samples in the linked story.
Why would you not want to highlight something in a sample? There is no reason why text in the sample would be less useful. A quick example that comes to mind is the foreword/introduction of a book that references a book for higher level treatment of the subject matter or another book for that is better suited for newcomers to a field.
