24p support has to do with maintaining the source material of film, as most films are shot in 24fps. Depending on the display in question (LCD, plasma, OLED, etc.), this format will either be left in 24fps, get up-sampled in multiples of 2-3 (48-72hz, i.e. advance mode on the Pioneer Kuros), or it will get converted to 30 or 60fps (or multiples thereof in the case of LCD displays i.e. 120hz-240hz) using frame interpolation/pull-down techniques, which can cause everything from excessive judder to the dreaded "soap opera effect" common to LCD/LED displays. If your source player and TV support true 24p, leave it on (or set it to auto so that it will engage when running film content and disengage when running video content). Hope this helps!
-David
-David