Thanks for your answer. I think I initially misunderstood the intended purpose of this tool (probably because of how many times the word RPC appears in the text) and assumed it was used for backend-to-backend communication, for which HTTP/JSON seemed extremely sub-optimal given the existing open-source solutions.
While it does make a lot more sense in the context of FE<->BE, the second part of your answer, where you speak about consistency of APIs across many services is still a little confusing, as it suggests BE<->BE communication again. Unless you have a huge, monolithic, internet/browser-facing service (which sounds rather undesirable) it's hard to imagine how browser<->middleware API could get out of hand to the point where it requires a dedicated unification framework.
Rephrasing my question - even though Conjure is not a commercial product (respect to Palantir for contributing to opensource) you must have had a target audience in mind for it: who is it? What exactly is the problem that Conjure is solving better than its existing alternatives?