CDMA needs precise sync for soft-handoff, but that's tangential to the rest. SONET explains it better, but even SONET is decades newer than the original requirements.

The T1 was introduced in 1962, having been developed in the 50s. T1 is self-clocking, in that the receiver recovers sync from the line (and there are mechanisms to guarantee enough ones-density to keep the clock recovery circuit working), so a point-to-point T1 circuit with analog on either end has no need for external timing -- one end can just free-run and the whole thing is fine.

But when you start connecting T1 (DS1)s together, moving DS0 signals between them ("time-slot interchange"), or bundling them into higher-rate signals (DS3/T3 and then up into SONET), it's utter chaos unless they're all synchronized.

There's considerable detail in the BSTJ archives, I could dig up some links later.