why use spring ??

Answer 1

You must go through Martin fowler's article http://martinfowler.com/articles/injection.html

Answer 2

Spring offers many services other than its IOC container. Declarative transactions, JMS, Timing/Scheduling, AOP, a MVC framework, etc.

Whether or not it's the right technology for your project depends on what exactly your project requirements are, and the environment in which it will be run.

Answer 3

Spring, among a lot of other things, is a library/framework for enabling dependency injection. You are welcome to code your own dependency injection library/framework and use that, thereby not requiring Spring.

If you create a powerful, annotation based, dependency injection library/framework, chances are, you'll come to something very similar to what spring already is.