You can watch this documentary to understand precisely what those tanks are for
Are those water meters I see? If so, it's likely you have an indirect supply - which makes sense for much of HK if your building is far(ish) from the WSD tank and higher up... i.e. you won't get enough pressure for your shower unless it comes from the roof.
It's not a must, but where supply comes via a transit tank, individual meters are often placed on the roof (after outlet to individual units). This is more likely in older buildings, whereas fancy new ones would have dedicated meter rooms.
Flushing water has no such meter - at least not individual ones, but there ought be a tank for that too. You might also find a tank for fire equipment, particularly if you have sprinklers in the building.
If you're bored you can go explore your roof again in a bit more detail.