It's not to warn the rest, it's even way cooler.
The smell attracts carnivores, and tells them "Hey there's some tasty herbivores over here" so they take care of the problem. The grass is snitching on the sheep.

Presumably that's why we like the smell of freshly mown grass, too (but such statements are impossible to prove in evolutionary biology).

Also worth saying that for animals, when someone nibbles off your arm, that's a serious injury which can strongly affect your survival chances. For plants, that's just a regular workday.

Kind of been my hardest lesson in keeping houseplants, too. Seemingly most plants need to be nibbled on (or ya know, get cut back), otherwise they will try to grow towards the sky and hurt themselves in the process.

I've killed two basil plants, because you look away for one second and they just grow half a meter tall. To support the weight, they become woody at the base. And eventually, they can't sustain the leaves at the top anymore, but when you cut them down to the woody part, they can't grow leaves on that anymore, so RIP... 🫠

There are heat sensitive ones that curl up.

It might reduce surface area or help survive overheating.

On the contrary I've seen one where one cell passes straight through another cell, making a hole. The cell that was passed through did not react at all and kept about its business afterwards, even regaining shape. Wild.

Pops and cracks from a stressed plant doesn't mean a physical sensation of pain is occurring.