The different settings are for if you need to mix songs with a wider gap (eg 140 BPM to 120) - if all of your songs are generally in the same range then the smaller the range the better, as the slider will be a bit more precise that way.
The master tempo button keeps the song staying at the same pitch (key) even when you adjust the tempo of it. If it's off, the key will adjust with the tempo, like a record (if you speed it up, vocals will sound high pitched)