Executive summary:
Program objects, also called classes, have advantages at the conceptual level
and at the implementation level. The central idea is that often you want to
think about and deal with the object as a whole, without having to fuss with
the internal details. Engineers call this the “black box” approach. You are
happy to drive a car without spending every moment thinking about the details
of how the engine works internally.
Programming in the modern style makes the code easier to write and easier to
read. It makes the code more reliable, more maintainable, more usable, and
more extensible.
The C++ string library is by itself enough reason to stop using plain C, but
there are many additional reasons.