It’s probably more accurate to say that English just uses a “condensed” or “rudimentary” form of the case system (The Cases in English | Department of Classics) in contrast to many other Indo-European languages (see, for example, for "Proto-Indo-European: Cases in Indo-European Languages: an article by Cyril Babaev
Proto-Indo-European knew eight or nine cases:
From a German or Latin perspective (in my case), the Russian case system is easy to understand.
The problem is rather, as Toby wrote, to be able to use the Russian cases fluently in a fast-paced conversation. And for that you can resort to the usual approaches: classic drills, SRS drills and / or CI.
Why does such a system exist?
My thesis is: it’s the same reason why
- word orders (SVO = subject - verb - object, etc.)
exist, namely, to reduce the overwhelming complexity of language units where everything can be coupled with everything.
It’s likely that (oral) communication would collapse instantly if human minds were confronted with such a linguistic “free play”. Therefore, morphosyntactic “constraints” have evolved in language-based communication processes.
In other words, word orders, case systems (or particle-based functional equivalents in Japanese, etc.) and collocations have evolved in all natural languages to make communication probable and easier.
But the price language learners have to pay is this: they have to internalize the word orders, countless cases and tens of thousands of collocations - if they want to be fluent in an L2 (similar to natives).
As soon as these constraints are internalized, language users can build an infinite number of context-dependent sentences, which means the linguistic complexity in discourses / texts can explode.
In short, the basic mechanism is here:
A general (unstructured) complexity must be reduced in order to be able to increase a more specific (structured) complexity. This is what all complex systems do… in this respect, oral communication based on natural languages is just one example.