This was used when you had a phone or device you couldn't rewire for A-lead control. Things like cordless phones, fax machines, and stuff of that nature.
I have my partner system behind my 1A2 system, so these work perfectly to give me A lead control when I'm on a partner phone, especially if I answer a call on a 1A2 set and place it on hold to take it on a partner set somewhere else. This will let me retrieve the call from hold on any other phone.
