Why Do Some Plants, Flowers And Shrubs Have Highly Fragrant Leaves...?

Plants and shrubs such as basil, lavender and rosemary all have highly fragrant leaves and I was wondering if there was a reason for this? Can anyone tell me why some plants this, whilst others don%26#039;t?

FlowersBirthday FlowersSympathy FlowersIn the case of herbs it is believed that the leaves are fragrant/scented to repel anything from eating them. This is the case with lavender which deer and rabbits won%26#039;t touch. Rosemary will be nibbled by deer but rabbits leave well alone and lemon balm is not eaten either. Furry leaves seem to be left alone too. Not a nice texture I guess.
Reply:A lot of geraniums are different scented as they are hybird plants. I would think all plants have same ability to produce a fragrance such as rose and lemon,,are many types and a neat one is the mint geranium.
Reply:Some need to attract insects.
Reply:to attract insects for pollination
Reply:Probably because different plants release different chemicals....make up