If your side project lingers on your personal site where nobody sees it, how would you ever know if people need what you're making? This is the typical "if I build it they'll come" pitfall.
You're basically talking about validating your idea before you spend any more time on it but if you truly care about validation, it's actually the opposite: you'll need to spend more time to validate your idea.
Your assumptions about my motivation are incorrect -- I need what I am making. That is why I make things. If other people also like it, then I turn it into something more. Sometimes.
It has built up into a little site with a few thousands users each month. Which I know is small potatoes to a lot of people, and doesn't hold a candle to my actual job. But it is plenty for me.
You're basically talking about validating your idea before you spend any more time on it but if you truly care about validation, it's actually the opposite: you'll need to spend more time to validate your idea.