It's actually the opposite of that. You use bits of stuff you have left over -- ends from vegetables, chicken carcasses, etc... Throw all that stuff in the freezer instead of throwing it away and use it for stock later.
In my own situation, I just wouldn't have enough vegetable trimmings to make stock -- I use the whole vegetable when I cook! For example, I never cut carrot tips off, and I only cut off the black part of the top, which is tiny and wouldn't really go in stock anyhow. Same kind of situation for celery, onions, peppers...