Having an extra ISO transformer and Atari transformer assembly around might just be a gamesaver one day.
ISO transformers RARELY go bad, but sometimes they do.
Sometimes you will get a Nintendo cabinet with a non-nintendo monitor in it. In that case it is wise to swap the isolation transformer, as those Nintendo monitors used a lower voltage than standard monitors, and non-nintendo replacement monitors sometimes have issues caused by the lower voltage.
As for the extra Atari transformer assembly. Having a spare one (specifically a spare big blue cap), can be very helpful in troubleshooting almost ANY Atari game from 1975 onward.
Now for the spare tube. I WISH I had one.
Someday you might get a game with a vented tube, or you might vent the darn thing yourself, a spare tube with burn, while worthless when selling, is a lifesaver when you need it.