This configuration is especially suitable for two units of musketeers exchanging fire if there is no distracting cavalry attack. Colonel Barriffe recommends this  for its stability and solidity while being able to deliver as much shot as other formations but: "...with half the danger."* 

That "half the danger" is of keen interest to the pikemen.  The musketeers shelter them from enemy fire.  They remain protected until the battle shifts to the point their services are required (in which situation the formation of musketeers may be reduced as illustrated in the animation). One can conjecture the Demi-Hearse was historically a favorite of pikemen.

The animation illustrates two methods of deploying the musketeers.  The commander may utilize other means, however, even a maneuver as simple as the files of musketeers firing and countermarching.

*Chapt. 76, Military Discipline...