StringBuilder Vs StringWriter/StringReader

I recently read that in StringWriter and StringReader are used for writing and reading from StringBuilder.

Well when I use StringBuilder Object, it looks to be a self sufficient class.

We have every way of reading and writing the StringBuilder, using StringBuilder.Append(), Insert(), Replace(), Remove() etc...

1. What is the possible use of StringWriter and StringReader, which cannot be done by StringBuilder itself?
2. What is the practical use of them?
3. What could be the possible reason they are not taking up Stream as the input (Because any other writer and reader are taking the stream as the Constructor parameter to operate on) but the StringBuilder?