Bags of frozen mixed vegetables are not precooked. If you dump these in your soup at the end to allow them to thaw, you will have very crunchy soup.

Contrariwise, canned vegetables (e.g., string beans) will have been sitting around in salty water at arbitrary temperatures for, um, how long has that can been hiding in the cupboard, anyway? And, thus, if put in at the beginning of cooking something, may have dissolved entirely by the end.