Functions and citizenship
In traditional structured languages like Pascal, functions and data are distinct. This is often inconvenient, so in C we got functions pointers and in C# we have delegates. These constructs go some way towards granting functions citizenship, in that we can treat functions as data in some limited ways.
In C++ there is another opportunity: Functors. A Functor is something that can be used as a function. Since C++ allows overloading operator(), this includes objects. Any class which overloads operator() is a Functor, since objects of that class can be called with function syntax.
Representing functions as objects opens up possibilities to store functions, pass them as arguments, etc. It also allows us to do something even more interesting: composing and sequencing Functors with generic algorithms where you pass in the concrete functions/Functors to be called.