> Can someone point me to an example where a class is more suitable than a function working on an object like an associative array?
I/O or resources.
Any time you're interacting with a stateful system like an API or a database, there's inevitable setup and maintenance to keep access to that resource sound. Additionally, you rarely care how that resource is kept coherent, you just need to access it at parts of your program. Without a class or some system that keeps some bookkeeping data around with methods, every function has to have that bookkeeping state passed in as a handle, and all of a sudden all functions need to know something about how setup works, as opposed to what it does. This is a rare instance of where namespaces + globals relative to those namespaces reduce complexity; even languages like Clojure have DI "objects" for resource access, and lots of languages that have IO monads are really a data structure over some state + functions.
This setup of objects as things you can't interact with directly--aka not data--is the original one in the literature. Once upon a time you needed to interact with a device driver or a mouse or a screen or a hard drive, and logically encapsulating that device was useful. You'd then interact via "messages" dispatched to the object which are effectively what methods are these days (the term "dispatch" is still used in the literature). It also makes mocking out that physical real-world thing easier.
Notably, objects never were meant as a way to represent data. I'm one of those functional programming weirdos that likes most things to be immutable, and I have simple stateless functions in an enclosing namespace. I pass simple structs (or case classes) of data around in those functions and it works well for me. But the I/O or externally interfacing parts of my program are always in specific places in my program, and they always use dependency injection, and it's always obvious what they are. They look like a sandwich where those objects are at the edges of the program (APIs/ingress and database/egress), but the meat of the program is in the inside and is boring pure functions.