Well, so the short answer would be, because I say so, look, I have the videos to show it, just look on my YouTube channel.
The long answer would be, Pure Water and Barrier only diminish the value of res if any character who uses them no longer cares about taking damage. If you've designed your game such that even your least resistant character only takes ~8 HP damage from a magic-using enemy, then yes, these items diminish the value of res. If your least resistant character takes ~20 HP damage from a magic-using enemy, then that's much different than if a more resistant character takes ~12 HP from a magic-using enemy. The former character takes 13+ HP damage per hit and the latter character takes 5+ HP damage per hit. This is a large difference and it results in the characters being able to do different things.
This video is the first that comes to mind of one that demonstrates what I mean (mute if you don't want to hear me talking):
Turn 1 PP: Saul uses Barrier on Perceval (13 +7 res).
Turn 1 EP: Windam attempts to berserk Perceval (54 hit; would've been 96 hit without Pure Water)
Turn 2 EP: Milady (6 res) gets hit by a Purge and a Bolting and is brought down to 4 HP.
Perceval gets hit by a Bolting and a Divine but also dodges a ton of stuff over the course of the chapter, so his HP definitely could've been much lower than the 32 HP that he ended with.
If we were to switch Milady and Perceval's roles in this situation, obviously it wouldn't work. +7 res Milady faces 96 hit vs. Berserk, which is a huge difference compared to Perceval's 54 hit. Milady would also have a much greater chance of dying, because keep in mind that Perceval took 19 HP damage after dodging a mage, a sage, and a knight (while carrying Roy, no less). So it seems clear that despite the usage of Barrier, Perceval's res advantage over Milady was significant.
A res difference is still a res difference, regardless of Pure Water or Barrier availability. Unless your magical enemies are so bad that a Pure Water boost on anyone invalidates them, then a unit with a res advantage will be able to do more things in more circumstances with a greater chance of success. The only thing that you accomplish by removing Pure Water and Barrier is to increase the value of the guy who has 20 res, but no one worth using will usually have 20 res, so this seems like an odd thing to buff.