What is the purpose of "chase planes"?

"Hobo" wrote in message

What is the purpose of "chase planes" that are always involved with
the testing of every new aircraft design?

Gathering visual data, taking promotional photos and videos, etc. Also
providing ground control with important information vis a viz aircraft
attitude, angle of attack, etc., should the pilot of the test aircraft
become incapacitated and unable to report his situation prior to a crash.
Or if the aircraft itself suffers some sort of degenerative structural
failure that the test pilot might not be immediately aware of, or a
fuel/hydraulic leak that a malfunctioning sensor isn't reading.

Essentially it's another set of eyes directly on the situation.

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