Why does Marcellus tell Horacio to speak to the ghost

They want to know why the ghost is stalking the ramparts. As an intelligent and skeptical individual Horatio relies on reason and measured judgment. He will "not let belief take a hold of him." So that is why Marcellus has "entreated him along." Being a scholar, Horatio should be able to get information from the ghost. When the ghost appears Marcellus says, "Thou art a scholar, speak to it Horatio." Then Bernardo say, "It would be spoke to." Again Marcellus prompts Horatio, "Question it, Horatio." Horatio then begins to question the ghost as to why it appears on the ramparts of Elsinore and more why it appears in the "warlike form" of the dead king.