where the cascade styles are useful?

Answer 1

If I catch what you mean,

(1) CSS itself is useful because you can set up an *external* style sheet to define the colors, fonts, locations, and other "appearance" aspects of a website, all in one place, to give all pages a consistent look and feel, and if you ever have to change them, you can change them in ONE place! Like, say, if you decide you want to use large Georgia font instead of smallish Verdana font in your website, or say, you want sidebars on the right instead of on the left, just go to that ONE file and change it!

(2) Cascade is useful because you can OVERRIDE the cascade by inserting in ONE file, overrides for the OVERALL style, by adding style statements in the section to change appearance of that ONE file.

(3) Cascade is useful because a, say, disabled user (color-blind?) can change your green colors to blue by using her/his own style sheet! Or by setting up her/his browser to use NO style sheet whatsoever!