I bet you figured that since you have a website, it probably works fine on all devices. But have you checked? And where and how would you?
A knowledgeable web developer will try and create sites that are cross-platform. In other words: viewable and functional in most browsers. I say most, because we can not realistically test in every one in existence. At least, not if we want to do anything else with our time. And there are many that most people have never heard of beyond what I think of as The Big 5: Internet Explorer, Firefox, Chrome, Opera and Safari.
All web browsers, because they are made by different companies and different people, interpret the code used to make a web page differently. The more fancy the site, the more we have to do to make the site cross-platform. A knowledgeable person will do their best to work the code so a site will function as much as possible, and look as good as possible, among lots of browsers. We also consider technology beyond your standard computer too, such as smart phones and iPads. We also think about your visitors who are blind and are using screen readers that tell them what is on the page.
Due to the way browsers are made and technology evolves, a web page may even be interpreted differently between different versions of the same browser too. For example, there is a difference in how we create sites for Internet Explorer 6 and Internet Explorer 8.
This is the difference between having a site done by a professional and having it done by a friend, child, or other person that does this as a hobby. Generally, the amateur has not learned that their code does not operate the same way in every system. They don’t know to test to make sure what they are creating is cross-platform. This is not bad per se. Not for a hobbyist site. But for your business, it could be disastrous. What happens if a sizable chunk of your audience is using a browser that your site is broken in?