For example, a freelancer might set up in business to write web content for companies in the travel industry.
There is nothing wrong with that, so long as you keep coming up with fresh ideas. New ways to market your business, new types of content to offer your clients, different types of clients to approach, new ideas on how to make your website or blog work harder. And so on.
At another level, I know a couple of freelancers who recently came up with a neat idea. Both are freelance writers and copywriters, and both have their own businesses and websites. But they got talking together and had a completely different idea.