It's a theoretical optimum location for measuring whether temperature at a weather station is likely to be representative of average temperature across the surrounding area, with requirements for flatness of land, vegetation of the appropriate type and height, no shade between sunrise and sunset, no windbreaks and distance from reflective surfaces.

Has nothing to do with its suitability for use in longitudinal study, because whilst a site next to a massive concrete windbreak might be a degree or two hotter than other locations in a region, unless the concrete windbreak is a recent phenomenon, it can't be the cause of temperatures at that site being higher in the 2020s than they were in the 2000s or 1970s...