Natural & other medicines
I've noticed that the overwhelming majority of herbal teas available on the market are blends. I actually prefer buying herbs in pure form and either drinking them on their own, or blending them myself. But there are a number of herbs that are common ingredients in blends that I find it is hard to purchase in pure form.
Some of the active ingredients in a herb can be easier to absorb from water than from food. It can also be possible to get a higher concentration in a tea than from eating it - for example, you could easily make a tea with 10 grams of a herb but you probably wouldn't want to eat 10 grams of it. There may be other benefits but I think these two are important.