I do that for my indoor seedlings only. Baby seedlings in potting soil indoors are very prone to a fungal condition called damping off, which kills them. Baby seedlings can't be allowed to dry out and indoors there's not a lot of air circulation. So continuous moisture and not enough air circulation leads to the fungus. And once you have the fungus you also tend to have fungus gnats. The larvae of those eat the roots of the seedlings. Very bad.
I just sprinkle a tiny bit (like a small pinch) of powdered cinnamon in a pitcher of water that I use to water with (bottom watering only, just pouring the water in the trays the seedlings sit in). And I keep a chamomile tea bag in the pitcher all the time, replacing it every few days or so, when it no longer has any scent of chamomile. You have to be more careful with the cinnamon, which can burn plants if used to excess. Chamomile doesn't, but I think the combination is more effective. For my use with my seedlings this has worked very well and using it, I no longer have any problems with either damping off or fungus gnats.
But I have not tried using it outdoors, for fungal diseases on adult plants. Don't know if that would work or not.
How I learned about all this was from reading here at Helpful Gardener!