I think it is to do with manufacturing process. Most low cost pots have a carbon track that has been deposited onto some kind of fibre board, which the wiper mechanism and cover attach onto. They are designed to be cheap and easy to put together. You can also get sealed pots, but they cost several times more:
The gap is handy for squirting cleaning fluid into, though.
The more common technical reason is for cooling with restricted convection air flow.
Many slider substrates are simply metalized films with power ratings of only 0.1 to 0.2W and the plastic film is a thermal insulator so adding more restricted air flow when operating near rated power, will raise the plastic film temp and risk burnout or fusing the element.