When you are using Public Switched Telephone (PSTN) line, you typically pay for the time you use: The more time you stay on the phone the more you'll pay. And you generally don't have the option of talking with more than one person at a time (or you can, but at increased cost).

With VOIP, you can talk all the time with any person you want (the requirement is that the other person has an internet connection), with no regard to distance, and you can talk with many people at the same time.