If you basically do the same job every day, showing up at 9 and leaving at 5, it wouldn't be for you. If you are working on multiple projects, it's a lot more useful. Especially if your employer has an SAP type time tracking system where the charge numbers are so ridiculously complicated. As a contractor for those kinds of companies, I've always struggled with keeping track of how much time I've spent on any given day among the projects I was responsible for.

Then there are other types of professionals, like lawyers, who have to keep track of multiple tasks on any given day for billing purposes.

Point being, there are still plenty of potential customers for time keeping apps. Maybe not enough to get rich, but enough to help make a living.