If a guy is a LCDR in the U.S. Navy, what may he gain from being sent as a graduate student to Ft. Leavenworth, Kansas?

Officers must complete Military Education Level-4, or Staff College, in order to be competitive for promotion to O5.

Each Service has it's own MEL4 level instruction and will have classes with students from other Services and other countries. The DoD itself has such an institution in Norfolk, VA, called the Joint Forces Staff College. It also is Joint and Combined

The Army Command and General Staff College is at Leavenworth. It is MEL4. Several Naval Officers go through Leavenworth each year. Since an Officer must do MEL4, he can do it anywhere. In some circles, it is considered prestigious to be selected to attend another Service's or countries staff college.

Following CGS, the Navy guy could stay for an additional year and go through the School of Advanced Military Studies, or SAMS. We call those graduates 'Jedi Knights'.

Sounds like you are writing a book.

JPME or CGS. Gives him a lot better chance of making O-5.