I'm not sure specifically as I haven't read it in detail. But generally, light novels often involve unique story settings, interesting characters, and some kind of adventure or development. It might be about a gacha - related world where there are various characters to be collected and adventures to be had within that infinite gacha system.
Well, it's not wrong as long as you're getting enjoyment and knowledge from them. But if it's affecting your other responsibilities, then maybe it could be seen as a problem.
Well, picking up the dungeon light novel for free can be wrong if it's against the law or the terms set by the rights holders. Usually, if there's no legal way to get it for free, it's not the right thing to do.