The 25 digits activation code is necessary to install and activate any software be it McAfee or any other antivirus solution. The activation code is something for which you actually pay for. Without this code, you will not be able to show the authority and authenticity of the product. To start using the product it is necessary to download and activate it which you cannot without having an unused product key.
This code makes sure that a user will not face any difficulty while using the product and gives him the idea that the product he is using is not a pirated one. The software with the product key implied on it is officially registered with the company who owns it and this tells us that the product is authentic. With a registered product, you get an opportunity to get the latest updates for the software.