I feel like the lighting of the beacons scene is kind of a microcosm of my issues with the LOTR films as a whole, in that:
Cinematically, it's absolutely gorgeous and stirring
The visuals are lifted even further by the score
It's a reference to a thing that is actually in the book, just highly re-contextualized (the beacons exist in the book and have already been lit, but serve a different function; it is the Red Arrow that is used to ask for Théoden's aid, with the specific remark that Denethor is asking for aid and not demanding it; the messenger who brought the arrow is caught and decapitated on his way back to Minas Tirith and so Denethor can't know if the message got out without using the palantír)
The lighting of the beacons in the films is tied into the story they're telling, in which basically all the NPCs other characters are much more self-doubting and self-sabotaging and it's up to Our Heroes to get them to do the right thing or the heroes just do it themselves (see Treebeard, see Théoden, see Faramir...)
Specifically, the necessity of lighting the beacons in the films is a direct byproduct of making film Denethor malicious and incredibly incompetent
The quiet, almost incidental tragedy of the messenger's death in war—not in a big battle, not in any glorious way at all, just this random guy being casually chased down and killed—is lost in favor of something dramatic and show-stopping and cool.
It is dramatic and show-stopping and cool! But sacrifices were definitely made in order to work it into the story at all and I think those sacrifices were very representative of the films' adaptational approach.
