Remember the old cliche warning us never to assume anything, well it's true! In order to get the most from your website and ensure that it's getting you the best results, you need to constantly challenge your assumptions. This means you need to run split tests. Take as an example, a particular test we ran recently for a client. The change was being made on an important page of the website. Even though we felt that it wouldn't have any negative effect on traffic and conversion, we needed to make sure. The results of the test were surprising in that by adding the button, we saw a 10% increase to the site.
This tells us two things, firstly that our assumption was wrong and secondly if we hadn't tested, we wouldn't have known that by adding the button we could get better results.
The bottom line is this...if you want to get the best results you can, then test everything you do.