When it is necessary to use Classes in PHP

Hello,

I am a wordpress plugin developer. I saw the source of many pluginsand many plugin use "classes" in them and some without classes.

Many huge coded plugins like "wp-postratings" does not use Classes and some small plugins use Classes.

edit: Since classes have private, public and protected access specifiers, i feel a taste of security Does it increase security?

Other than PHP does other languages require classes ?

Is it compulsory to use Classes ?

I need some suggestions.