Because it's good for your beard dammit!
Beard oil moisturizes your facial hair and the skin underneath providing the necessary nutrients to give it the soft and healthy appearance it deserves.
Beard oil can also help prevent the itchiness and flakes you would incur when your facial skin and hair are dry.
Beard oil will also leave your beard smelling damn good.
Be warned though, heavy petting and sniffing of your beard by others may occur when you oil your beard.