This is how many artists have worked. They make something for themself, and one day they show it to someone else ... or they just get the urge to share it more widely, often without the hope that anyone will really be interested. Or they keep it for themself.
I think Tolkien is in that group, for example. But don't get the wrong idea from an extreme outlier: much of the time, others aren't interested, or not many are. Sometimes, nobody is interested until after you've forgotten about it or passed away. Who cares? That's one reason you need to make it for yourself. Also, I think that otherwise it provides much less expression and insight into another person, which is at the core of art. There is a fundamental human need to 'externalize the imagination'.