Because God knows all everything we are asked to do by Him is for a great purpose we may not know now. Nephi understood the necessity of obtaining the plates to help preserve the language of their fathers and pass down the information on the plates from generation to generation. And by obtaining the records it would preserve them from the destruction of Jerusalem.
The records then could be used to keep the language unchanged and the words of the prophets from long a ago safe from corruption. This would help teach generations to come and do so much more as we know and understand better here in this day and time.
Reason, God know all thing. Everything Himself finish ask us do mean important us maybe not understand or know now. Nephi finish understand need get metal book save language past father and generation++ recieve can. get metal book avoid destroy J-city can.
Metal book use how? save language same++ word from prophet past save change can. Help teach children future do more, understand now can.