What's the font-lock-mode for in emacs?

Answer 1

From the emacs help on function font-lock-mode:

Toggle Font Lock mode.
With arg, turn Font Lock mode off if and only if arg is a non-positive
number; if arg is nil, toggle Font Lock mode; anything else turns Font
Lock on.
(Font Lock is also known as "syntax highlighting".)

Answer 2

• Is this expected behavior? With font-lock mode, am I supposed not to change any font?

Yes. You have to customize the font faces and colors using customize-face command (change colors starting with "Font Lock"), or use a theme engine like color-theme.

• What's the good for font-lock mode?

You can customize and extend font-lock to your need as this is the case for all things in emacs. This is the basic system for syntax highlighting that is used by the majority of the emacs users.

You have to learn how to customize it to your needs (C-h i m Emacs RET m Font Lock RET )