An ISP is not necessary to use a network adapter on your PC. However, as the others have mentioned, you won't be able to access the Internet without an ISP. Additionally, the majority of the computers from the past 2 decades come with a built-in Ethernet card. So unless the built-in card doesn't work, or your PC doesn't have one for whatever reason, you don't need to buy an Ethernet card. However, if you want Wi-Fi, then you might need to buy a Wi-Fi card. Most modern desktops come with them built-in.