Your visitors do not care why your website is slow. And they do not have time, nor patience to wait for the page that does not load quickly enough. WhySlow? offers you peace of mind by giving you reports about the health of your URL performance. Images, Javascript and content from 3rd party providers may affect perfomance of your site dramatically. And if you care about positive experience of the visitors of your site, you need to be able to identify the sources of slowdowns as soon as possible. Regular checks from WhySlow not only will alert you when your site performance is not up to par, but also will give you the ability to diagnose why it is slow and trend the performance.

Another reason to use WhySlow? Let's say your site is well optimized, but not heavily visited. That means that at times your site will be visited when the cache is not initialized. Every time a visitor gets to the site that has no cache content that visitor has to wait longer. Whyslow solution - have whyslow be the first visitor initializing the cache! This way when the real customer comes in they don't have to wait extra for the content to load!

OK, sure your website could use some performance tweaks. How are you going to know if the change you make is helpful or hurtful to the performance?
That is when you really want to monitor with Why Slow©. Sure it is good to find out when 3rd party Apps are a problem. But when tuning your site, you have to know if you are helping or hurting. You have to be able to track and review the history of how fast your site loads. That way, you can have a better idea of when to back out that bad change, and keep the good one.