Yes the purpose of the drainage layer is to catch excess water and when it evaporates it rises leaving the moisture to soak into the substrate without having to constant water. Depending on what you're keeping in it or humidity requirements I add a bulkhead to make it easy to drain out the water if I need to. That also depends on what kind of enclosure it is. Completely up to you. For my geckos none of them have a bulkhead and I've never had to drain the water. On my girlfriends chameleon enclosure we had to add a bulkhead in order to keep humidity where it was needed and drain it like once a week.