When you convert from 30fps to 24fps, you have far fewer frames to encode for the same amount of "video". This means a smaller file size (or, the same file size with better quality.)

For example, I encoded my 18 second lightsaber scene as 24 and 30fps. The 24fps segment was 3 MBs smaller. Over a 2-hour movie, that's approxiamtely 300 Mbs of wasted space.

Point being, the presentation of the movie will not change - you are simply compressing it to fit into a smaller package (which can easily be expanded and recreated digitally.)