Why we have to use getcomponent instead of a simple reference ?

myclass a = new myclass();

myclass a = getcomponent<scriptname>();

what is the difference ?

First creates a new class of type myclass.

Second one creates a reference to an existing class on the object and I don’t think it’ll work without a cast.

Becuase with a simple reference you are calling a simple script, just script, which doesn’t control any gameobject. So you need to use getcomponent to have a script of the gameobject and control variables of that gameobjects.