What is the purpuse of thors on a rosebush?

Answer 1

To protect the roses from animals/bugs which may pluck/eat the rose instead of pollinate it (which is the reason for it's bright color, for bees).

Answer 2

Thorns on a rosebush serve as a defense mechanism to protect the plant from being eaten by animals. They also help the rosebush to climb and support itself as it grows.