Every SaaS business starts with an idea. Even though most likely throughout the lifecycle of a SaaS startup, the idea will change and evolve, you still need to start with an idea.
Often people come up with ideas as they work in their area of expertise. They discover a problem people are facing, come up with a solution that could potentially solve that problem, and believe people might pay for that solution.
Many startups cannot define the problem or solution well, which causes many problems as they progress.