|What I want to know is in which case do I need the knowledge of polymorphism in game programming?|
Any situation in which you might want several objects of different types, but which all share some common properties or behaviour, so they derive from some common type, or conform to some common interface.
Do you have different types of monster, all of which can attack, but which attack in different ways? Polymorphism might be what you need.
Do you have different types of objects, all of which can be used, but which have different effects when used? Polymorphism might be what you need.
There's no single area in which you will or won't need polymorphism. It's a fundamental pattern in OO design - so fundamental, that we don't even tend to call it a "pattern", because it's existed since before the word "pattern" came into vogue. It's widely applicable to a huge range of situations.
It's one of the absolute fundamentals of OO design. You really should try and find a good tutorial source, because it's pretty crucial to understand it.