I just want to know if it's necessary that my entry and exit points in the Schengen Area are the same. For example, I decided to enter in Amsterdam and exit from Milan. Is there any issue?
As you can check here, your Schengen visa allows you move freely between any of the twenty-six Schengen member states including the Netherlands and Italy. There is no problem in your proposed itinerary.
Note: Certain flights within the Schengen area are treated as international flights for operational reasons, for example. If you have a single-entry Schengen visa and intend to travel by air within the Schengen area, you will want to ensure that your "intra-Schengen" flight does not fall under this category. In practice, as far as you stick to European carriers such as KLM, Alitalia, EasyJet or RyanAir, this should not be a problem.
In addition to the answer by SgrA, remember that you may not misrepresent your itinerary in the application, and that you should avoid the appearance that you misrepresented it.
So if you plan to travel from Amsterdam to Milan, say so in your application and there is no problem. If you change travel plans afterwards, it must be plausible that you changed them only after the visa was granted and that they there is no fundamental change to the premise of your trip.