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The necessity for telling LR the offset is due to the ancient EXIF spec that didn't allow time zones to be recorded for photos' capture times.
GPX tracklogs record their timestamps in UTC (GMT), whereas most cameras record whatever time you've set the camera's clock to. (Until recently, the EXIF spec didn't even include a field for time zone.) When LR compares the time recorded in a tracklog with the time recorded in a photo, LR converts the times in the tracklog to the time zone of the computer running LR, and it assumes the photo's capture time is also in that time zone.
If the camera's clock was set based on the same time zone as your computer, you don't need to use LR's Set Time Zone Offset command. For example, suppose your computer is in San Francisco and set to PST (UTC - 8), and you set your camera's clock in PST. LR will consider a photo whose capture time is "14:00" to be 14:00 PST (22:00 UTC). It will then look for a point in the tracklog with time close to 14:00 PST (22:00 UTC).
But suppose you travel to New York, you set your camera to the local time, EST (UTC - 5), and you took a photo at 14:00 EST (19:00 UTC). That photo will have 14:00 recorded in it (but with no time zone). You go home and load the tracklog and photo into LR on a computer set to PST (UTC - 8). LR will assume that the photo was taken at 14:00 PST (22:00 UTC) rather than the actual 14:00 EST (19:00 UTC), and it will look for a point in the tracklog with time close to 14:00 PST (22:00 UTC).
That's not what you want, but there's no way for LR to know which time zone a photo was taken in. So you need to use the Set Time Zone Offset command to specify the offset between your computer's time zone and the time zone where you took the photo (more precisely, where you set your camera's clock).
The most recent EXIF spec (2.31, July, 2016) finally included an EXIF field for cameras to record the time zone of the capture time. But most cameras in use still don't record that field, even though many allow you to pick the time zone when you set the clock, and many know the time zone automatically (e.g. smart phones and cameras with GPS units). Some cameras record the time zone in proprietary locations.
Given Adobe's lack of attention to metadata, Library, and the Map module for the past many years, and given how clunky the catalog internal architecture is for storing capture times, I think it's unlikely they'll ever change the map module to use any time zone recorded in EXIF. But you never know -- you could file a feature request in the official Adobe feedback forum: Lightroom Classic CC | Photoshop Family Customer Community