The advantage of measuring biosecurity is that you make biosecurity quantitative and tangible. by measuring biosecurity you can see how good the biosecurity on a farm is performing and where the weak and strong points are. In systems that allow benchmarking (comparison to other farms) you can also compare your results with the results of others which allows you even better to see how well you are performig.
Measuring biosecurity also gives you the opportunity to measure the improvements made. If there are some weak points in specific components of the biosecurity you can develop a plan to improve these and later on, by means of a new biosecurity audit, you can measure (and demonstrate in numbers) which improvements are made.