The most common reasons cats eat plants like roses include soothing teething or simply playing. Eating plants in small quantities is normal cat behavior and usually isn't a problem unless the plant is poisonous. Roses are non-toxic in small quantities, provided the stems and thorns are removed.
Cats do not have the ability to digest plant foods efficiently, so while it is possible that eating plants may supply extra nutrients, this has not been proven as a reason why cats eat plants. Sometimes a cat will chew plants after learning that this attracts its owner's attention. A cat may also chew leaves after pouncing on them in play.