How to create a .h class implementation file?

Question

C++ / C#

There is a file Sortable.h

#pragma once

template<typename T>
class Sortable
{
public:
  virtual int compare(T *t) = 0;
  Sortable();
  ~Sortable();
};

How and in general why write a .cpp implementation file to it ???
Compiler always writes (create or not) to use class template requires argument list template
member function not defined in Sortable