1 Nephi 19:21

for he surely did shew unto the prophets of old, all things concerning them; and also, he did shew unto many, concerning us; wherefore, it must needs be, that we know concerning them, for they are written upon the plates of brass.

He showed the old prophets everything about them. He also showed many people things about us. So we need to know about them because their stories are written on the brass plates.

He certainly showed the ancient prophets everything about them. He also showed many prophets about us. So we had to know about them, since it’s written on the brass plates.