What does "what can it buy us" mean? When is it used?
We should try to understand, what for are these theories created, as Americans say, what can it buy us.
The idiom "What does that buy us?" means, "What do we get from this?" or "What can we actually use these for?" In the context of your example, the person asks what real-world benefit is gained from these academic theories, to refute the suggestion that they have no useful application.
It can be called an American expression because of this country's focus on money, so many think we only care about the financial value of things (and care little for pure academic progress). This may or may not be true, of course, but nevertheless the stereotype exists.
A similar expression is "to get down to dollars and cents" which means to assess the real-world benefit from a particular expense. In your example, you could say:
We should try to understand, what for are these theories created -- if we get down to dollars and sense, what practical purpose do they serve?
I'm not sure this is a commonly used phrase (only 6 results on Google, one of which is this question) with any meaning other than the literal.
So someone might say "We made five dollars today!" and someone else might respond unenthusiastically "Sure, but what can it buy us?", implying that five dollars is not going to be able to buy much.
It's definitely not a common phrase, as others have mentioned
I have heard this in the context of "what progress does this allow us to make", especially in regards to academic work.
Out of context I would expect this to be with regards to money, for example walking into a shop and asking "what would £5 buy us?" but in context that's clearly not what's meant