Vegetable stocks give any soup more full-bodied flavor. The stock can come from water saved after cooking vegetables, from rinsing out cooking pots with small amounts of water, from fresh vegetables or from scraps. In my world using-what-comes-to-hand provides unexpected depths of flavor— but there is no recipe for it.
Clasically, meat stocks utilize the virtue of onions, celery, and carrots, as well as parsley and dried thyme, and with good reason: they round out or enhance flavors. So vegetarian stocks often utilize the same basics (for the same good reason). And you can vary the stock depending on the soup you are preparing.