It's useful for habits, communication, and planning. What time do banks close, Usually 5pm. When's lunch rush hour, between 11am and 1pm. When do events like sports or primetime television or the local bowling league start, maybe 6-7pm. When do kids go to school, 7am.

You have all these references in your head about when people go about their day. Phrases like "man, the busy season started and I ended up working until 10pm last night" don't need clarification that you worked about 5 hours over.

If you ever left your native timezone, you can just look at your phone or a local clock and make assumptions about the world around you. If the clock reads 7pm, you know you still have time to hit a restaurant. Move two timezones east, and now 7pm means you don't have time.

That's the exact same thing.. we could say that you start work at 8 and finish at 16 half the year, and work 9-17 the other half. We've just decided that it's more practical to say that you always work 8-16, and then shift the clock to keep in sync with when there is more light outside.