So that ,that class is accessible from some other class/file
as well

First of all, it only has to be the same when the class is 
public. And there is no explicit reason for that, it's just 
a convention that came along with old versions of java and 
people got used to it... They say it's because of the 
limited capabilities of the compiler to compile 
dependencies.

If u dont keep public in front of class that is not 
accessible by third party packages. But if u keep public 
infront of class that is accessible by third party 
packages. 

Problem:  class CheckServlet extends HttpServlet
     {---- }
 Now u want to executed this servlet class in server 
(Tomcat,WebSphear,WebLogic) an error will come .Because the 
above class is comes under default access modifier. 

Ans: public class CheckServlet extends HttpServlet
     {-------} 
 This class services will executed in any server(either 
Tomcat,WebLogic etc.,). Because The class is visible in 
third party class packages like container packages.