This one is pretty self-explanatory. Completing the Demigod Rune Tablet is an astronomical task that requires a lot of time and/or zeny investment.
Why is this good? So completing the demigod rune tablet feels more satisfying. And besides, the damage output you dish out from completing it puts you on a league above other dps players. Making you embody the term "demigod".
What could be reason why this is a bad idea? Absolutely nothing.