In the case where you never go anywhere, how is that different from giving them your precise location? They very likely know enough other things about you to deanonymize a postal code into your actual address.
In the case where you do go places, the difference is that you get a weather report for where you are, rather than where you live. When I'm at my girlfriend's house, and I check the weather report, I want to know what the weather's going to be like tomorrow at my girlfriend's house, not at my house. And when I'm at the office and considering where to go for lunch, I want to know whether it's going to be raining at noon in the area of town around my office.
In the case where you do go places, the difference is that you get a weather report for where you are, rather than where you live. When I'm at my girlfriend's house, and I check the weather report, I want to know what the weather's going to be like tomorrow at my girlfriend's house, not at my house. And when I'm at the office and considering where to go for lunch, I want to know whether it's going to be raining at noon in the area of town around my office.