I like to use the analogy of a freeway. If you have a multi-lane freeway that’s the one big pipe coming into your organization via public internet, that means your voice traffic, your video traffic, applications and world wide web browsing all comes into the freeway all at once. It's all running through the lanes and people are jocking for positions and trying to get through.
Imagine the NCAA Men's Basketball Tournament is in full swing and everybody is trying to watch the games online. All of a sudden, the video traffic goes through the roof. Video is hogging all of the lanes. If you’re trying to squeeze a little bit of a voice conversation into there, it's going to be a problem. If video is hogging everything, you’re not going to get a really good voice performance. It’s hard to fit that voice traffic into the broader, clogged up freeway.
However, with a non-public internet connection like an MPLS network you get quality of service (QoS). This QoS creates a special lane on your freeway that is optimized for a certain type of traffic. This way you can handle a surge of video traffic because you audio is using a special lane.
That’s why you’d really want to run your voice traffic, at least for your business, over something that has a MPLS enabled network where you can optimize that traffic and control quality for just that kind of traffic. A public internet connection simply doesn't provide those options.