What do "they" mean by the phrase - "this team lacks leadership" or "someone needs to step up to the plate on defense and show some leadership"?

I can see how the quarterback is an obvious leadership role: plays are called and changed from this position, and the accompanying cast on offense must respond in unison for the play to be completed successfully.

The defense is a different matter it would seem. Each defensive player is reacting to the offensive alignments, right? In other words, if the defensive players knew their assignments and responsibilities, how to line-up in reaction to pre-snap formation the offense is showing, then why the need for a "leader" on defense (like Rolando McClain with Alabama last year).

Why can't each player on defense be "an army of one"? Why should they need a Rolando McClain to flap his arms around and point them to where they should be lined up or who they should be covering?

I can see the rookies needing some assistance early in the year, but as the season progresses there shouldn't be as big a need for a leader on defense.

I've never played organized football, so I'll admit that I could be well off base with the above observations.

(PS - Or by the term leader do they mean someone that gets the guys pumped up and focused, sort of like Tebow with his sideline speechifying and gesticulations?)