In the opinon of programers why is string builder used instead of a string?

When you add something to a string, a new address in the memory is being allocated. The previous string is copied to a new location with the newly added part.In case of StringBuilder which keeps the same position in the memory without performing the copy operation. That means string is inefficient.

Example:

string str = "hi";

// create a new string instance instead of changing the old one

str += "hai1";

str += "hai2";

//StringBuilder working

StringBuilder sb = new StringBuilder("");

sb.Append("ha1");

sb.Append("hai2 ");

string str = sb.ToString();