[Lapsa]

advantage is when you manage to couple things at composition root (which is the very beginning of your application). that way there is one convenient place where coupling takes place.

read "Dependency injection in .Net" by Mark Seemann. it's the best resource to understand wtf DI is all about and what are benefits.

[giulianob]

Yes even if you don't program .NET, the first half of the book is just about manual DI principles that could apply to almost any language. His blog also has good stuff about DI.