I see 2 options:
1.) There are enemies we come across that we can't feasibly kill at our current level. I prefer this option. However, in this case the enemy "level" needs to be labeled, at least for the standard difficulties, as cgexile said. Otherwise there will be an overwhelming amount of complaints from people getting into fights with enemies they can't beat without knowing they couldn't win beforehand.
2.) If we follow the default playthrough, ~every enemy we come across should be beatable. (boring). In this case, enemy "level" is basically irrelevant and could be hidden.
Red dragon: For single monsters, there's a big discrepancy between CR and what level party can beat it. Venomfang, a young green dragon in LMoP, is technically CR 8. However, a level 5 party could probably beat it with ~50% chance. I've heard stories of level 4 parties killing him. So, if the red dragon in BG3 is actually a young red dragon, combined with the buffs BG3 gives PCs through height/jump/etc, a "level" rating of 5 or 6 is probably appropriate which is not that far off from the listed level 4...
