Answer: Without tugboats, large ships entering a port might accidentally hit something nearby.
Step-by-step explanation:
Large ships going through dangerous areas need Escort Boats to accompany them and since a tugboat is an Escort Boat that guides ships coming into or leaving a port, it is safe to assume that tug boats are needed to accompany large ships into a harbour.
One of the main reasons a large ship would need to be accompanied into a harbor is so that it doesn't hit anything (like guiding an 18 - wheeler into the loading Bay of a supermarket) by accident due to its size. Tugboats are meant to accompany large boats so it is safe to assume that Tugboats guide big ships into the harbor in an attempt to ensure that such boats don't hit others