I have to wonder what practical purpose is involved. I'd hate to know how much that cost.

Ask you shall receive. Here is a response from that AI overview thing I've heard so much about:

Rocket sleds provide a crucial, ground-based platform for high-speed testing, simulating extreme conditions for military hardware (missiles, ejection seats, guidance systems) and studying the effects of rapid acceleration/deceleration on humans and equipment, bridging the gap between lab tests and actual flight, saving costs, and ensuring safety by testing dangerous scenarios without risking piloted aircraft or expensive flight tests. They are used for aerodynamics, impact studies, and validating new technologies for aerospace and defense.

__________________
1986 300SDL, 362K
1984 300D, 138K