Is "Only Use Offline Maps" setting effective for Data Saving?

No one here provided a good explanation to your exact question. I believe the answer you’re looking for is that offline maps downloads only the map itself and chooses the best route based on predicted traffic. So as not to use any mobile data. Online uses data not only to load in the map but also to show real time traffic updates to better predict your route. Turning on offline maps essentially disable cellular data from Maps. There is no hybrid option yet where Maps can use your cached offline map and also use real time predicted traffic. It’s one or the other. Therefore, Maps is not smart enough to use your offline map and switch to online maps once you leave your cached map area. I agree that this system is impractical and cannot be intuitively used like most of Apple’s features.

Edit: Additionally, I believe that off-line maps is not used whatsoever when online maps is on. The only current benefit to offline maps is that it will automatically switch to offline maps only when your phone detects that you have no cellular connection. But even with 1 bar of super weak connection, it will still try to use online maps so even that aspect is not very seamless.

Only use offline is an option to not use data while using maps. If you venture outside of an area downloaded to your phone you will not have working maps.

The "Only Use Offline Maps" setting is designed for situations where you have a limited or no data connection. When this setting is enabled, Apple Maps will only use the offline maps that you have downloaded, even if you are connected to Wi-Fi or cellular data. This can be useful if you are traveling to an area with poor or no internet coverage, or if you want to avoid using any data for navigation.

However, if you want Apple Maps to use the most up-to-date information, you should keep the "Only Use Offline Maps" setting turned off. This will allow Apple Maps to use real-time traffic data, incident reports, and other information to provide you with the most accurate and efficient navigation.

I understand how it works, but it's just plain old bad UX in my opinion. Phone should be always looking for downloaded data and leveraging it, me as an user should not care whether I have turned on some option or not, if I have previously downloaded the data.

If the concern is out-of-sync data, there could be an option "automatically sync downloaded maps when Wi-FI" or whatever if data loading is concern (roaming etc cases).

I am pleased the evolution of Maps, but to close the final gaps with competition, PM's who are driving this product should just try to use it on various network conditions themselves sometimes....