the Chaining switch, why?

[Babybearjs]

  after playing with and rewiring 2 20 button phones, I am asking you guys about the purpose of the chaining switch. based on the schematics, its wired in series with the T/R/A leads and I don't quite understand the purpose of a dual switching circuit. you have the switchook, which already controls the line and A-Lead circuit, plus the line key button... so, why the chaining key? an extra line of defense? or just another switch to get in the way? what have you to say about this?