Because String is an immutable type, the variable value can change but the original data value has to be discarded from memory and a new data value has to be created in memory. StringBuilder, on the other hand, was designed for multiple string operations.
So, if you have a case where there is a loop and you have to manipulate data in a string more than maybe 10 times, the code will be better off using StringBuilder. But do use plain old String concatenation if you want to do something simple like this:
string sometext = "You should use " + "StringBuilder" + " in large loops or you are just loopy.";