Usability testing should always be part of product development. Ideally, it is conducted before release, but in practice it can be useful at different stages. It's worth pointing out that, "testing" isn't a goal in itself. Without knowing exactly what you want to measure, you risk ending up with feedback you can't interpret or act on. Every usability study can focus on a different aspect of the experience, and the right metrics depend on your research goals.
Let's be brutally honest: your brilliant SaaS idea is probably a fantasy. You've got a beautiful mockup, a clever name, and a Trello board full of features that you just know people will love. But here's the cold, hard truth that kills 9 out of 10 startups: 1 Nobody cares about your features. They care about their problems. And right now, you're just guessing what those problems are.
User research insights often fail to drive organizational change because they are not connected to business objectives, making it difficult for stakeholders to recognize their potential impact.