There seems to be no reason to clear the level. I fought off hordes that I should have left alone. The only reason is to make sure there isn't another power up, and there seemed to only be one once.
Yeah, there is no reason to clear the level except finding the power up. But sometimes there is a second power up by chance. And there are also wells that can be found.
Going up without exploring will make it harder to defeat the endboss.
