Steam distillation
Some substances cannot survive being heated to their own boiling point. This method gets them to vaporise well below it.
The problem
We know that volatile compounds are present in some plant parts — cinnamon, cloves, citronella, nutmeg and cardamom are a few examples. To separate these compounds out, it is difficult to raise the temperature steadily up to their boiling point. At temperatures near their boiling point these compounds can decompose and be destroyed, or be converted into other compounds. So the heat is supplied to the mixture by steam instead.
Why steam lowers the temperature needed
Whether a compound mixes with water decides which way its boiling point moves when water is added.
Compounds that mix well with water
When a compound that mixes well with water is mixed with water, the boiling point of that mixture rises above the boiling point of water.
Compounds that do not mix with water
When a compound that does not mix well with water is added to water, the boiling point of that mixture falls below the boiling point of water.
Most scented compounds do not mix well with water, and they have boiling points higher than the boiling point of water. Inside living cells they exist mixed with water. So they fall into the second case — and that is the whole trick.
Figure to be added
What comes over, and what you do with it
When heat is supplied to these mixtures by steam, both the water and the scented oil leave as a vapour mixture at a temperature below the boiling point of water (100 °C). When the escaping mixture is cooled, the water and the scented oil do not mix, so they separate into two layers. They can therefore be separated easily.
Steam distillation exists because the alternative destroys the thing you were trying to collect.