Usual start time 9.30 am. Like others it lets people get back from school run and also rush hour has died down a bit by then.
It also allows for emergency jobs, little jobs etc to be slotted in at 8.30 which again is then before the worst of the rush. If I have no emergency jobs then I can do some paperwork etc.
It suits me to do earlier as well, say 7.45 to 8.00 in some cases. For example, single people who leave for work just after 8.00. I can arrive before they leave, be shown around the job (including where the coffee is) be paid and given a key to push back through the door once I have left. Customer comes home in the evening to dry carpets and I am not arriving early evening when by which time I am tired and wanting to get home.
In run up to Christmas all the 8.30 slots go then it's the 7.30 slots and once I was known to start at 6.30 on a dark morning in Christmas week. Didn't get home that day till 22.30; well knackered...but extremely well paid!