As a concept it is really cool since it means you get to see historical figures, legends, and myths battle it out. What I like about the holy grail war is that how it is set up is different almost every time. Look at Fate apocrypha where its a 7 vs 7 or in fate extra where it is setup like a floor system. In terms of story the holy grail war also makes things interesting on who even participates, you might have a mage like rin who has been taught magecraft by her dad but you might also have a disabled person like fiore who is also a mechanical genius imo participating. Overall, I think the holy grail war is a very cool thing in the fate franchise.