Why? It explains a lot about society and it holds perfectly. That's the definition of a good definition. What's your definition of economic classes?

There is an internal contradiction within our capitalist systems, some people live by selling their labors for wages, and some by leasing their capital for labor (to get more capital). Evidently those two interests are opposed, and ergo leads to issues when one side gains two much power over the other, because the power imbalance is very much not self-correcting.

Classes are used to refer to these groups of opposing intersts. There would be no need for classes if there was no conflict. Things only exist in their opposition to other things.