It is, as you suggest, a resonator.

It has two functions: Firstly silencing, but mainly to equalise pressure waves and the harmonics on the intake side of the throttle body. When the moving column of air in the intake hits a closed valve there is a sudden compression as the air is compressed and bounces backwards. This causes pulses, or resonance. The result is a temporary clogging, or high pressure area in your intake tube that can limit airflow.

Adding a resonator forces air bouncing back to collect in the chamber, slowing the backwards pulse and equalising pressure. This then allows fresh air to flow towards the engine from the airbox without it having to fight against the reverse pressure wave.

Theoretically adding an intake resonator (air chamber) makes the engine quieter and more powerful.

The physics is of course a lot more complicated than this, but hopefully it gives you some idea of it's purpose.

Interestingly the Z doesn't have one, but that could well be due to the different design of airbox meaning that intake noise, resonance or back pressure are less of a problem?

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