When you purchase McAfee product, you get a subscription that lets you install, activate and use the software on a certain number of computer/PCs/laptop for a certain period of time. The time of your subscription depends on your purchase, but usually begins when you activate your McAfee product.
If you want to purchase your product from the McAfee officially website, you don’t have to activate your software.
If you want to purchase your product from a retail store, you must activate after installing your product.
However, if you purchase your product through McAfee partner (like Dell or Acer), you must activate after installing your product.