Here's the problem:
Websites break. They break a lot. Constant innovation and change means that your website is almost never working at 100%.
If the success of your business depends heavily on your website then you know the pain that issues like a broken checkout, non-functioning buttons, mobile display problems and more can bring. It's your worst nightmare, it can happen at any time and these types of issues often lie undetected for days, weeks and even months - all the time hurting your conversions and costing you money.
If only you had someone or something watching over your site 24/7...