fignewtron was here

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:

Code