Think of the continuous line current as a circuit continuity indicator that also
served as wetting current for the un-soldered (twisted together) copper wire
circuits, and kept corrosion at bay (that's why the idle signal voltage is
negative wrt. ground - it counters the natural electrochemical potential that
appears against wet things (including ground) that touch the line and this
prolongs the life of the copper wires). Later it also served other purposes
(several keys could key the circuit, when connected in series or shorting to
ground). The wetting current issue is still valid in rural areas where wires
often chaff and bounce in the wind causing bearable noises in the phone audio
circuit. Without the wetting current (pure ac coupling) tens of copper oxide
junctions created at the twisted wire splice points would oppose signal passage.

The 'key when shorting to ground' feature was used quite a lot (party line
including hooking into the line anywhere between stations using a ladder and a
hook, with a portable key that keyed to a ground point created by driving a
stake into the ground). RS232 receiver compatibility with driving to GND
(instead of going negative) may date back to those times. Phone lines (POTS) are
based on an expansion of the same principle and in the beginning they shared
wiring and switching with the then existent telegraph wires. It is still
possible to use a straight key and sounder on any POTS line that is straight
wired (static mechanical or relay switch and/or party line).

Peter


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