I don't know much about this subject ( technical) but ctcss tones allow for more even coverage.+Kevin Reid
Exmple of the need:
In California there are 2 repeaters one is the W6CX on Mt . Diablo near concord (147.0600Mhz, Pl 100, 100W) and one on the Nevada side of Lake Tahoe (147.6600, Pl 123, 100W) 130 mi away both are coordinated but at night they used to interfere with each other. The Nevada one used to be on Pl 100 for 2 weeks when anyone keyed either repeater the hundred Watts of the other repeater would override the incoming ham and both would stay keyed until they timed out.
If they were on the same frequency the signals would interfere. I was told by a grandfather that it was like when you put a microphone to close to a speaker.