What is the role of passing by reference when you do not modify variables?

Question

Are these references(&) just an issue of saving memory or idioms or is there a reason to why statements like these use references when passing by copy would accomplish the same thing.

template <class T>
bool testGreater (const T& one, const T& two);

or an object declaration such as this:

Cars BMW (const Engine&); //where engine is a class

What is the function of passing by reference when you do not need to modify the item passed?

Answer 1

When you pass by value you must make a copy of the object. Depending on what object is used to instantiate the template, this can be expensive. (or impossible. Some objects are not copyable)