g'day Jabawocky,
There have been a few instances recently of start clearances.
One instance had to do with extended single runway operations at Sydney, meaning the sectors feeding the traffic into Sydney become overloaded, hence the start clearances. No use letting aircraft taking off if the holding stack is already full.
Another instance was due to a thunderstorm overhead Sydney airport itself. This rendered the airport unusable for half an hour, leading to similar issues with traffic holding as mentioned above.
These are only a couple of instances that I am familiar with, and they were directly related to traffic management.
Thats not to say that there were other instances that were not related to traffic management, if you've got some examples, do divulge