When was the purpose of bone marrow discovered?
Answer: In the late 1800s, by a few different guys building on each other.
I personally had no idea, but a little searching around found this little gem, which seems up our alley: The origins of bone marrow as the seedbed of our blood: from antiquity to the time of Osler.  It's only ten pages so definitely worth a read, but a few passages of note:


Neumann and Bizzozero... are credited with making the initial observations leading to the recognition of the marrow as the seat of blood formation... In his report published in October 1868, Neumann described the presence of nucleated red blood cells in the bone marrow sap of humans and rabbits by squeezing bone. He was the first to conclude that during postembryonic life, erythropoiesis was taking place in the marrow.

However, Neumann concluded erroneously that the proliferation of marrow cells occurred inside the blood vessels of the marrow; the concept of marrow sinusoids was not coined until Minot's description in 1901.



It later says that, in the mid-1880s Osler stated

that there can be no doubt that nucleated red cells originate in the bone marrow...

It's actually a pretty interesting read, all in all, and like all of the best discoveries in science, it seems like a few people built on each others' work over the years, with a bunch sort of figuring it out around the same time.