One reason to hold a teleconference is that all the key players cannot be present in one room. There are also times when not only the participants cannot gather, the planners can't always get together either!
Let's say you need to have a meeting of the minds with people in a faraway branch of your company. You may need to coordinate with people in that office to go over the agenda and goals for the meeting. You will likely be able to get all of this figured out through phone calls and e-mails, but it might be better for you to have a quick chat via teleconference to go over last minute details.
It's a cliché, but there often needs to be a meeting before the meeting. Since you have to get everything set for the teleconference anyway, why not get together with your fellow planners right beforehand? Using a Pre-Conference setting, meeting planners can discuss logistics right before a call and early arrivers will not be able to hear a thing because their lines will still be muted. Do a sound check, calm jitters, do all that you need to do right before the main conference starts.
You have the technology already in hand, why not set aside a little extra time to make sure things go smoothly?