Some plants grow sharp thorns and hard skin for this reason

Plants have their own daily battle which is the constant unending battle for survival with animals.

Because plants cannot run away from danger like how we do with our legs which they don't have, they need to build a natural protection to stay away from herbivores. All the herbivores see is food, they really don't care about the plant life, I doubt they even think plants are alive like how the carnivores see their prey as alive.

Thankfully some plants skin act as protective armor, being rigid and hard to tear. This makes it hard for an animals to bite through the skin. When an animal bite into the plant, its skin stops them from entering inside. Plants have thorns for extra protection, since thorns can puncture and badly injure the mouth of any animal ignoring the danger signs.

Animals can learn that some plants are not good for eating if the plant has thorns, especially for smaller animals.

But nothing is for free, it may not be money but it still costs the plant to develop this protective skin, since some sugars that would have contributed to growing larger leaves will be used to form the thorns, or the skin itself.

If sugar and nutrients are limiting, then the animals will be foraging more, making the plant develop protective skin for protecting its seeds during its growth.

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