It starts with the assumption that somehow you know it’s 4:25am in another country across the world. How do you know this? Because you are aware there are difference in time of day between one place and the next. Armed with that knowledge you are required to do math about appropriate calling times regardless of whether there are time zones or not.
But it’s even worse than that. Most of us don’t just take a crapshoot and guess whether it’s normal waking hours and shoot off a random phone call. Knowing when it’s a good time to call someone is more important and often you arrange calls before hand via text or email. With time zones the conversation (via text message or email I assume) goes something like this: Q: “When can is a good time to call you?” A: “Any time between 8am and 10am is fine”... now you have a math problem to solve to know when you can call. Without time zones, the answer is immediately clear and relevant to you.
Quite often you don’t even know what time zone the other person is in.
Likewise, businesses can post their hours publicly and people don’t have to do math to figure out whether they can call AmCo in the UK at 9:45.