I’m making lentil soup but I don’t want to make an extra trip to the store. Why not just increase the vegetables the recipe calls for and use water and seasoning?
No reason. You can produce a perfectly acceptable product using the method you described. The boxed vegetable broths I’ve tried are just celery and onion water anyway.
Well, when I boil noodles, usually to make Asian food, I boil them in Vegetable broth. It just adds a lovely extra layer of flavour to the dish. But I’m not boiling veggies, in the production of these dishes, so making my own vegetable broth would add a very labourius and time consuming step!
The OPs technique would work.
However, you’d get more flavor if you actually made a broth. The idea of a broth is that you get the goodness out of the vegetables, resulting in overly-cooked, mushy, tasteless vegetables.
It’s pretty easy to make veggy broth, not laborious at all. Here’s a decent recipe, but really, you can put just about any combo of onion/leek/parsley/potato/mushroom/celery/carrot/etc in a pot, simmer it for an hour or so, and you’ll have decent stock.