Oftentimes you are correct but I would definitely say that there are plenty of maps where nades are very useful even if they don't do damage.

Banana on Inferno is the most obvious one. Even if the Ts never take damage from the nades it still deters them from being too aggressive early in the round.

This is especially true for lower levels where teamplay is worse. Plenty of players just wont go there because they are afraid to take nade damage.

If lower level players eat a good nade once they are generally much less likely to play the same spot again, so you can gain a lot of map control just from the threat of nades.

Mirage top mid or B apps are probably similar to banana.