Ray Lovinggood wrote
Of course, to us it seems, it doesn't matter if the
instructor is on the ground watching or in a towplane
towing or on the space station: he can't do anything
to help if something goes awry.

Exactly.

It's a dumb requirement, and it shows a total lack of understanding of
(or disregard for) the purpose of solo flight. Why does the private
ticket require solo time? What can the student possibly practice solo
that he can't practice with me in the back?

The purpose of solo flight is to impress upon the student the
life-and-death nature of his decisions. It is a chance for him to
make his own mistakes, and deal with them. It is an essential process
not in the development of skill, which can be developed just as well
(actually better) in dual instruction, but of judgment.

When you tell a student that he can't fly unless there is an
instructor on the field, you are sending a clear and unmistakable
message. You're telling the student that his decision to launch is
subject to review and may be overruled by that instructor. You are
therefore encouraging him to launch even if he's not sure it's a good
idea - because he knows (correctly or otherwise) that if it's a bad
idea, the instructor will overrule him. You can tell him otherwise
until you're blue in the face, but actions speak louder than words.

Michael