On a conventional lathe the part only rotates. Cross drilling, a keyway or a milled flat means removing the part from the chuck and setting it up on a mill. Every new setup means a new dimensional reference.
The C-axis makes the spindle's angular position controllable. The part is locked at the required angle and milled or drilled with driven tooling, without ever leaving the chuck.
Three things follow: fewer setups, shorter lead time and — most importantly — no tolerance drift. The concentricity risk introduced by a second clamping disappears entirely.
On small batches this difference can reach half the cost, because in small runs the real cost is setup time, not cutting time.