From what I remember from writing an undergrad history paper on these dudes, it was used for lots of things such as a treatment for chlamydia (or another STI - I don't remember exactly). These dudes were banging their way across America, especially the black slave they brought along as apparently the locals thought he had big magic.
I'm not condoning any of this sort of colonialism - just clarifying that these dudes probably single-handedly introduced some new STIs to whole populations, and they were dragging their leaky mercury-riddled dicks along in their boats.