All wounds have bacteria in them, but unless you've got certain comorbidities, your body is really good at using its own mechanisms to handle that, as long as all the waste products (dead white blood cells, dead tissue cells, loose bacterial cells) have someplace to go. When a wound is "open" like this, they get sloughed off the surface and absorbed by the bandages. No big deal (from your body's perspective). But if the wound is closed, all that stuff just collects in it, and you get an abscess which prevents further healing, causes pain and inflammation, and can lead to sepsis if the bacteria gets into adjacent blood vessels. It's like the difference between taking your trash out on a regular basis or keeping all of that trash in your kitchen as it rots.