Question: Lot of questions has been already asked about the differences between string and string builder and most of the people suggest that string builder is faster than string. I am curious to know if string builder is too good so why string is there? Moreover, can some body give me an example where string will be more usefull than string builder?

Answer: 
StringBuilder is, as its name suggested, is used to *build* strings, whereas strings are immutable character-values. Use strings when you want to pass around character data, use StringBuilder when you want to manipulate character data.
In brief, it's a trade-off between **mutable** and **immutable**

See this:[StringBuilder vs String](http://channel9.msdn.com/forums/TechOff/14294-C-string-vs-StringBuilder/)