If you want to participate in a financial or other business system, you need to observe their timekeeping system, which is generally UTC. If you say “well OUR country is using domestic time”, that’s fine, but business is still going to be conducted in UTC. You can use that clock, or use your own and risk things happening at the wrong time. Kind of like when you’re in different time zones and forget to account for that and show up to a party an hour late.
But the real thing is high-frequency trading. Instead of buying and selling days, weeks, or longer periods apart, trades are made with large sums based on market movements in milliseconds. If you can see that a stock is jumping upwards by a few cents over a short time, that doesn’t sound like much, until you put $1,000 on a 1% gain and make a quick $10. That doesn’t sound like much, but do that thousands of times a day, every day, and you can make a lot of money.