You're right.

It wasn't done for the planetary science. (That said, having actual trained geology field students -- and one professional geologist -- on the Moon in person was very, very nice).

Also worth noting that the state of the art of robotics in 1960 meant having live astronauts set up the various pieces of equipment left there meant more accurate set-up of more instruments and more complex instruments.

However.

The penultimate reason was political; to score points in world opinion. The US and the Soviet Union were each posing and primping and trying to get as many small developing nations to go to "their side" as possible. Looking like the masters of science and technology was one way to attract these nations. And the Soviets had a leg up on that with Sputnik.

The ultimate reason was to apply a kick to the pants to America, specifically science education, higher education, and the aerospace infrastructure. American kids were cutting classes and doing miserably in science compared to the rest of the world (sound familiar?) and American industry was turning out giant poorly-built automobiles and losing in electronics and miniaturization and a hundred other places to western Europe and Japan. (This should ALSO sound familiar!)

We could use another inspiring, national program now.