In response to:
Thinking about how application state evolves over time is difficult, and is responsible for all sorts of pernicious bugs. If you can avoid statefulness, you should ... but sometimes you just can't. In these cases, Classes give us a way to limit and control how state can change, making it easier to reason about.
In other words, placing state in a private variable, and limiting the possible state transitions with a restricted set of public methods (public setters don't count) can dramatically limit the combinatorial explosion that makes reasoning about state evolution so difficult. To the extent that this explosion is limited, classes help us to think and reason about how the state of our application evolves over time, reducing the occurrence and severity of bugs in our system.
The discussion of using classes as an OO domain modelling language is a separate question, and is (as the above article insinuates) is bedevilled with domain-model impedance mismatches resulting in unsatisfactory levels of induced-accidental-complexity.