10Ω series resistor in output of headphones amplifier
It is there for many reasons, but we can't really know what was in the head of the designer when this was drawn.

The output is unstable with capacitive load. The resistor isolates the output capacitance from op-amp.

Short circuit protection. It protects both the op-amp and headphones.

Crude volume limiter.

Op-amp output does not like to be driven into saturation. Saturation load is 25 ohms according to the datasheet. With 10 ohm in series, it can drive 16 ohm headphones without problems.