Pick the wrong one and you will either end up with a rope that is inadequate for the job, or more expensive than it need be. In most applications high strength and low stretch are priorities. Anchor warps are the obvious example but there is a place for some shock absorption in other places too, like trapeze wire adjusters. Six materials are commonly used to make ropes. These fibres can be used lone or in combination to produce ropes of widely differing properties and price.
Farmers use it for bailer twine.
From a sailor's point of view polypropylene has the great advantage of being less dense than water. Not only does it float, but it refuses to absorb water too. Unfortunately it is not very strong and doesn't offer much resistance to stretch. Left outside in the sun it deteriorates quickly. Despite its many apparent weaknesses, polypropylene finds many applications on dinghies and yachts.