Master tempo is a feature that has been around for a while on most modern CDJs, players and software. It is a feature that "locks" and preserves the original pitch of a track no matter what playback rate is used. When you speed up a track without master tempo on, it plays back faster and as such the frequency of everything will go up. Opposite occurs when you slow the track down, the song will be perceived lower. This is why pitch faders are named as they are.
This becomes a real issue when you are making pitch adjustments as there will be an audible change in pitch and therefore key. The audience will hear it if it is fast enough. By using master tempo, no matter what you do to the speed of the track, the key will remain the same. It really is a blessing to harmonic mixing.
Of course there is issues with it. Let's say you decide to drastically reduce the pitch of a digital track, perhaps to half. You are essentially halving the resolution of the track. A problem with low bitrate tracks especially. The more you try and alter the speed, the more the computer will have to work to maintain the original key.
Higher quality tracks with good bitrates will reduce but not solve the issue. And there is very little that can be done. Provided you mix close to the original tempo, there will be little issue. Often drastic tempo changes sound bad anyway before you even get to the audio quality so it is an issue that is not often encountered.
So with all this said, why isn't master tempo always on? Why don't we save on cost and remove the button altogether? Well there are a few reasons you may want to disable it.
Firstly with scratch DJs, you will see many using the pitch fader to change the pitch of a sample and control the notes to make a melody, JFB manages it perfectly in the video below.
If you are a devout traditional turntablist who is a complete technophobe, you may not want there to be a master tempo as you have learned to just adapt and manage without it.
Really however it is another tool to make the DJs life easier in the hope that more impressive mixing can be achieved. There will be people who hate it and many more who rely on it and in the end it is really just worth trying it on and off to see what you prefer.