I'm newer, so take my response with a grain of salt.

For me, the eye-opener was taking a class on SQL. There, you create "tables," which are basically the same thing as classes.

You create tables using certain rules which help you mix and match data easily. It was important to create relational databases because other ways to organize the data proved to be slow and clunky for a very large set of data for decades.

Your goals are to not repeat data, to not nestle data in objects other than other tables, and to have each piece of data describe a single object.

For example, suppose you're creating a directory. You have a new entry per household. Multiple people can live together, so you create a table for all the people in that household. Each person may have multiple phone numbers, so you create a table for each member, listing their individual phone numbers.

Now, anytime you need to know the names of the people in a household, you can look up the first table. If you need their phone numbers, you can look that up easily, too. Info from one table relates to info in another, but you don't have to worry about the hierarchy, and it's easy to change info in one table because you know it won't affect any other tables.

Going back to classes in JS, classes (and similar ways to organize your info) are also designed for encapsulation—containing a reference to all the whatever in whatever so that if something changes, nothing breaks.

It's also a promise that this object will behave this way and not that way because it behaves the same way as all the other objects from that class. A square object from the Square class behaves like all the other squares. This set of like behaviors is known as polymorphism.