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The top vent will help keep the smoke circulating, and provide a way to avoid stale smoke, as in creosote. The bottom vents are designed to help control the cabinet temperature. If you open them, you will usually get a hotter burn using more oxygen. I hope this helps you.
You have to have vents to allow oxygen in. Otherwise, oxygen would be depleted and your fire would go out (electric smokers excepted). Vents in the top and bottom are used to tune your fire/smoke to your desires.
Electrics still need vents for circulation purposes.
I am wandering the same thing. I have two side vents and one top vent. Last time I played around with the lower vents to see what would happen. I couldn't tell a difference. The only difference is that when I had them opened my wood chunks would let off more white smoke. When they are closed I get more TBS. Other than that I couldn't tell a difference. Is there one that I am missing. I have a perfect flame propane.