Check out this video featuring Michael Phelps and coach Bob Bowman explaining the advantages of training with the MP 'Focus' Snorkel:
Centre snorkels are a great training tool for intermediate to advanced swimmers. There is something to be said about the advantages of developing your breath control and breathing muscles (intercostal). The snorkel provides a degree of resistance, meaning that every time you breathe out you're training your lungs, and all the muscles that help with breathing, to be stronger/better than they were before.
The main advantage to swimmer's snorkels, such as the MP Focus Snorkel, is that you're more free to focus on your stroke and forget about your breathing. Most swimmers find that this clears away some mental space so that they can better think about what the rest of their body is doing during the stroke and make steps to iron out any weak areas.
Michael Phelps and Bob Bowman have worked with Aqua Sphere to develop this snorkel to the highest standards and create their own, more refined version of a classic training aid.