The main difference between struts & spring MVC is about the difference between Aspect Oriented Programming (AOP) & Object oriented programming (OOP).
Spring makes application loosely coupled by using Dependency Injection.The core of the Spring Framework is the IoC container.
OOP can do everything that AOP does but different approach. In other word, AOP complements OOP by providing another way of thinking about program structure.
Practically, when you want to apply same changes for many files. It should be exhausted work with Struts to add same code for tons of files. Instead Spring write new changes somewhere else and inject to the files.
Some related terminologies of AOP is cross-cutting concerns, Aspect, Dependency Injection...