Because they could not fight the Phoenicians by ship, whom were good at naval battle. The Romans were good at land battle. So, while on their ship, they invented the grappling hook to latch onto their enemy's ship and pulled their ship close enough to board the enemy ship for close--combat.
I was mind blown by this discovery. Cause it makes so much sense! It's good to know! Cause, ever since 3rd century BC, pirates have adapted the tool.
Below is a picture of Romans attacking an Egyptian ship:
My Remmington Pirates use it in the 1700s with all the other pirates and naval sailors using it too.
Below is from Assassins Creed Black Flag: Can you find the grappling hook?