It depends on what your own goals are. E.g. if your goal is to ‘solve a problem’ or ‘help someone do something’, then you evaluate it on those parameters.
However, if your goal is to build a successful, profitable startup, then you have to evaluate if the concept has a good business case around it. Start with the value proposition and evaluate if that is meaningful to a certain set of people. Assess how much they are willing to pay for it. Then check if the costs of delivering that value proposition is lower than the cost that the customer is willing to pay. And if the answers are positive, then assess if there are enough number of customers that will make this a meaningful sized business for you, and for your investors if you are thinking of raising funding from investors.
Most importantly, despite a yes on all the above, evaluate if this is something that you really, really are excited about doing. If not, no matter how strong the business case, it is just not worth it.