I was talking with a friend and from my understanding, milliwatts are intended to be used for line loss/quality measurements in conjunction with frequency meters/testers. there are also tone sweeps that can be used to measure frequency response. Toners are intended to be used for tracing pairs when you need to isolate or locate a pair in a residence, cross-connect (junction box), at a splice on a pole, etc... Toners are used in conjunction with an induction probe that can detect the tone to identify the selected pair.