Encapsulation allows you to formalize your interfaces, separating levels of abstraction (i.e. "application logic accesses IO code only in this and this way").
This in turn, allows you to change the implementation of a module (the data and algorithms inside the module) without changing the interface (and affecting client code).
This ability to modify modules independently of each other, improves your ability to measure your performance and make predictions in a project's deadlines.
It also allows you to test modules separately and reuse them in other projects (because encapsulation also lowers inter-dependencies and improves modularity of your code).
Not enforcing encapsulation tends to lead to a project's failure (the problem grows with the complexity of the project).
