The Papp's mute is an indespendible tool for the piano tuner. At some point, you are going to want to mute off one pr two strings and you wil have no room. The papp's mute allows the technician to target specific strings to mute when there is very little room. 

With most mutes you can mute the right or left string, or the right two or left two. The Papp's mute can also do this, and in very tight spaces. Wat must mutes don't allow you to do is to mute just the centre string. The Papp's mute easily does this.

The following video describes how the Papp's mute is used in the Basic Piano Tuning Manual. If you are interested in a more accurate and precise aural tuning method, consider the Go A.P.E. System.