Use case for more than two breadboard binding posts?

Question

I understand the use for two binding posts on a breadboard: namely to connect the + and - of a DC power supply. But what it the purpose of breadboards with three binding posts?

In this tutorial I read: "Typically, you only need to connect a power and ground wire from the posts to the breadboard. If you need an alternate power source, you can use the third post." But would I not need a third and fourth binding post to connect a second power supply?

So how do you make use of just a third binding post?

Answer 1

Some components need a positive and negative supply voltage, and also ground, eg OpAmps. This can be achieved with voltage-splitters or just plain old resistors. For basic dc electronics you won't really need dual-supply.
Or if you need for example 15V and 5V you could also use the three points, but thats probably a bit messy.