A spyglass or telescope is a tool used for looking over long distances, useful on board ships for navigation and looking for land as well as spotting other vessels.
Most captains will not be without one while at sea.
When placed on the monkey statue outside Rum Roger's House it would reflect the sun and shine a light into the building helping one to find a secret button with which to open a trap door.
If the player attempts to open it, Guybrush will comment that "This is not Monkey Island 1". (needs verification)