Trump doesn’t understand the question because he doesn’t understand doing things for the betterment of anyone but himself.
For most of history, you didn’t ask “what’s in it for me” when the king/prime minister/ The Church/ or President came asking (country irrelevant). That’s a relatively new luxury due to perspective of the digital age and disagreements with (the US) Government due to transparency.
For most of history “what’s in it for you” was actually getting fed and clothed better than the average peasant. Serving the king was what was in it because you didn’t have to sleep in pig shit and milk the cows every morning. You’d actually get fed for mealtimes instead of playing the barter game all summer and fall just to have enough food to store in salt barrels for winter. And even better, if you tickled enough enemy hearts with your pointy stick there WAS some land and money for you, provided you survived.
Some countries through history also revere their veterans (with actual respect and benefits) so military service itself was the honor. While I understand it’s a dramatization -the beginning of Disney’s Mulan is a great display of it. Her father is it is '60s or '70s and has already served once and has a bad leg. The emperor sends out a call for war and the guards show up in town. When they call his name he sets aside his cane and picks up the summons because that’s what you did. It is what was expected of him and he did it without complaint.