I think freelancers and contractors also have an advantage in that they're exposed to a far wider set of business problems when compared to a developer working

I think freelancers and contractors also have an advantage in that they're exposed to a far wider set of business problems when compared to a developer working as an employee. When you're exposed to a bigger set of problems, you're more likely to see something that you can provide a solution to. And when you see similar challenges happening again and again, you start to get a sense of what problems need a software solution. If only one client has a specific problem, it might not be amenable to a SaaS solution. But if you see the same problem occurring over and over across multiple clients, you might just have a good SaaS idea on your hands.