Your tailgate is specifically designed to allow water out of your truck bed. However, if you cover your truck but leave the tailgate un-sealed you will still get water and debris in your bed as you drive.
If you watched your truck in a wind tunnel you would see a large swirl of air at the rear. This happens as the air moving over the top or your truck meets the air moving under your truck. That swirl of air carries water, road dirt, salt, sleet and whatever else it picks up along the way. Some of that will end up in your bed if your tailgate isn't sealed. Conversely, you don't want to seal your tailgate if you don't have a tonneau as there would be no way for rain water to escape and without a tonneau there is air flowing through the gaps in your tailgate minimizing the dirt carried by the swirl.
Our TrailSeal Tailgate Gasket is designed specifically for tailgates. It seals all gaps around the perimeter of the tailgate to help keep dust, rain and snow out of your truck bed.