The explanation makes sense if Thanatos relates it to the armies fighting on Ares's battlefields. They are made of men taking different roles among the whole.

"Hm, my initial assumption is still correct." Thanatos tries to stifle the urge to yawn once more. If only he could shift like normal. He'd be back resting on the train instead of taking the long trek back. "You are someone to turn to for an emergency heal."