It was standard coding practices if you were selling games - to prevent people from hacking the license key stuff. Or by cheating software to prevent detection

It was standard coding practices if you were selling games - to prevent people from hacking the license key stuff. Or by cheating software to prevent detection by anti-cheat software. One of the hooks built into Windows is "is a debugger connected" helps the virus/cheat code determine if it is "safe" to run or not.