BBQ Gaskets are essential to keep smoke and heat locked in during slow cooking, they create a better seal between the lid and main kettle.
This modification is particularly useful in colder places like here in the UK, where we are often battling to keep the heat in and get the most out of our coals, as windy or cold conditions demand much more from the fuel source.
You can see on the picture this will seal several Weber Smokey Mountains (our gasket is black, white is used for that illustration)
You can also seal the Oklahoma Joe's, between the fire box, the cooking chamber lid, firebox lid and chimney.
I did find that it wasn't a good idea applying gasket to the top section of the Oklahoma Joe lids closest to the hinges, this created a clearance issue, but the remaining sides are fine, doing this will give you a great seal with minimum leaks.
Another observation when sealing an offset, was I pulled the gasket quite tight as I pushed it on, this made it slightly thinner which sorted any further clearance issues down the sides and bottoms of the door.
This is a 15ft roll!