Meaning of “he is necessary”

I am quoting a sentence from "East of Eden" by Steinbeck.

He told his son of the sad dignity that can belong to a soldier, how he is necessary in the light of all the failures of man—the penalty of our frailties.

I understood it roughly, but first time I have met a statement such as "…he is necessary…".

Does it mean that "how important to have the sad dignity for a soldier…."