Why use sans-serif at all?

When reading in some American university text books (the first example that comes to my mind being Young and Freedman's University Physics), I notice how much it uses sans-serif all over the place, for headers, figure captions, graphics, etc., practically everywhere except the main text. The design is so complex that there is no doubt in my mind that a professional designer has been involved. Yet what strikes me is how ugly I actually think it looks; having to go in and out of "sans mode" all the time (as well as adjusting to different text sizes, fonts, shapes, weights, text colours, and background colours, for that matter) simply disturbs my eye. I cannot remember the last time I even used sans fonts myself.

Can someone provide me with a good design-based reason why, when typesetting a book with main text in serif, using sans occasionally is a good idea? As a book designer, which positive effects would you say that using sans can have? (Because, sorry, but I find them hard to see.)