If you consider a novel a work of art, then creating it is an act of artistic creation that has some intrinsic value. There is a benefit to expressing yourself carefully, to telling a story that says something about the world that you think needs to be said. Those outcomes are "significant" even if the novel has a small audience.

Once you discuss rulesets and guidelines, it's probably rather a craft. Just because it looks like art from the past century doesn't make it art.

Most art takes craft.

> How to write a novel that is actually a worthwhile read?

There's no silver bullet for that, but one promising approach is to honestly try to write something you'd really like to read yourself.

> Why should you invest your time into an endeavour that will most likely fail and have no significant outcome?

The answer to this fully depends on your personal definition of a significant outcome. Some people consider working on open-source projects a waste of time. Other people think the same about working for a corporation. For some, the only outcome worth working for is noticeably and unambiguously making the world a better place. Others (the happy ones) just do things "for the hell of it".