They're only worth making for your own enjoyment, or to have some friends or total strangers come hang out. 99% of boards serve no real purpose and just die out. But the good ones endure despite it all.
Like, I used to run one about Serial Experiments Lain (this was way before Lainchan or whatever so at the time it was unique) and it had no real purpose, but myself and friends would post and that was enough to justify it. I also continue to run some random, obscure boards on different web servers. I don't advertise them or expect them to get any attention, so I just leave them up and if anyone finds it, they can say hi. They get maybe 3-4 posts a year when someone just happens to stumble upon the website.