From 7110.65, 3−7−5. PRECISION APPROACH CRITICAL AREA,
2. GLIDESLOPE CRITICAL AREA,
b. Operators commonly conduct “coupled” or
“autoland” approaches to satisfy maintenance,
training, or reliability program requirements.
Promptly issue an advisory if the critical area will not
be protected when an arriving aircraft advises that a
“coupled,” “CATIII,” “autoland,” or similar type
approach will be conducted and the official weather
observation indicates a ceiling of 800 feet or more, or
the visibility is 2 miles or more.
Even when the weather is clear, it gives the Tower a heads-up so they can keep airplanes and airport vehicles out of the area where signals might be disturbed in a way that would contribute to an unsuccessful test.
Bob