What can only users tell you about being “smooth-sailin’”?
If you want to weed out errors that you don’t know about – and there are errors that you don’t know about – you need to ask your users to point them out.
Why? Because there’s no replicating the variables at work when different people, using different devices, on different internet connections, from different parts of the planet, at different times of the day use your website.
It just can’t be done. There are aspects of your site that will appear to work fine to everyone in your company (you’re all on the same network, probably using identical devices), which are actually a nightmare for a significant number of your users.
And unless you regularly test with random groups of people, your site is where errors and glitches will hide out from the UX police.