in times of emergencies. For example the death of Netero.
It's more like that Kurapika is the de-facto boss of the Nostrade Mafia Family.
I'm gonna guess that Nobunaga and Kurapika had met up at some point and ended up beating or killing the guy. The way he was drawn in the panel looked as if he had just gone through something big and maybe was at the shrine giving his reassurance that the revenge of the spiders is one step closer.
As for Leorio, they were together at one point of the story so maybe he was calling Kurapika to see if he was done with his business. They must have been doing something together. They're probably going to end up meeting each other. I can't guess any other reason why he would call him.
I'm a bit vague but thats my general consensus. lol
i was thinking how togashi would connect gon's history with kurapika's and spiders history. now that ging said something about going to another world or some thing like this, and that he said "the chosen ones from a certain era have RECORDS of when they went to the other side" made me think about kuroro. if i remember well kuroro likes to steal OLD BOOKS, at least one of the spiders said that(chapter 71 page 18)... so i think kuroro want to go to another world too, and consequently meeting gon or ging in the process.