I have never read the trilogy so I don’t know how well Gandalf and Saruman were explained therein, but in the film trilogy viewers get no real sense that these characters are not human and immortal and essentially “angels” working for the god of the universe. And Sauron as a “fallen angel”.
Even when Gandalf returns from certain death I don’t think it is really explained, it just happens. I’ve talked to several film only viewers and they just pegged them as wizards, and Sauron as just your average evil overlord.
I just find it kind of essential info for even understanding what is going on, but granted it would ruin the cliffhanger of FOTR and Gandalf reappearing in TTT.
EDIT:It also explains some odd things in the film if you know Gandalf can’t directly intervene but only guide the mortals to victory.