Java: what are IOEXceptions in BufferedReader's readLine() for?

I can "fix" the below exception with a try-catch loop but I cannot understand the reason.

1. Why does the part "in.readLine()" continuosly ignite IOExceptions?
2. What is really the purpose of throwing such exceptions, the goal probably not just more side effects?

Code and IOExceptions

$ javac ReadLineTest.java 
ReadLineTest.java:9: unreported exception java.io.IOException; must be caught or declared to be thrown
  while((s=in.readLine())!=null){
                      ^
1 error
$ cat ReadLineTest.java 
import java.io.*;
import java.util.*;

public class ReadLineTest {
 public static void main(String[] args) {
  String s;
  BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
  // WHY IOException here?
  while((s=in.readLine())!=null){
   System.out.println(s);
  }
 }
}