Take a look at the following code.
const string input = "interesting";
bool comparison = input.ToUpper() == "INTERESTING";
Console.WriteLine("These things are equal: " + comparison);
Let’s imagine that input is actually user input or some value we get from an API. That’s going to print out These things are equal: True right? Right?!
Well not if you live in Turkey. Or more accurately, not if the current culture of your operating system is tr-TR (which is likely if you live in Turkey).