You'll find that some of the gravity fed charcoal cookers such as Backwoods offer convection fans as options but they're by no means essential.
The fan on the Traeger is mainly for provision of oxygen to the fire pit which is very small and runs like a mini furnace. Most offsets do not use fans.
You see the use of fans on air intakes in systems like the stoker and BBQ Guru but these are for fire control rather than moving heat and smoke around.
On a typical bullet like a WSM you'll find the temperatures stay relatively even on the two cooking levels and as long as you have the correct amount of exhaust (very important) smoke passes through the chamber and exits, you don't get an accumulation of smoke at the top. With inadequate exhaust this may happen and that's when you get creosote build up in your smoker .. yuk