Intervals in small independent cinimas still exist on long films, this is due to the fact that the projector has only 1 reel on it and can only hold so many feet of film, so on long films the intermission is there so the projectionist can rewind the current reel and then change it for the second one.
Big multiplexes often use more than one projector I believe, and this means its seemless for the change in reels.
Thats the technical bit lol