There are three good reasons to register your project. First is to have it in our system, so that when it launches (especially if you tell us that it's out), we can put it on our 'Powered by Wwise' games page. Second, is that if your release is a commercial one, it is required to register it, since the License Agreement is part of the registration. Even for a free license (budget under $150K and under 500 sounds), if your game is commercial, you must have a License Agreement in place. Third is that this is the only way for us to get you a formal license key. So for an official Starter License (free), the license key will let you jump from 200 sound limit to 500 sound limit. For a larger game, this is the only way to get access to other platforms, removing the sound limit, trying out premium plug-ins and more.
If your game is non-commercial and under 200 sounds, then you technically do NOT need to register it (our registration flow will actually tell you this if you choose those options), but they as I mentioned there's no way to get it into the gallery.
Hope that explains things,