Lando is literally the perfect person for Max to be fighting for the title.
It means Lando can't afford to take risks which slows him down.
Lando wants to win, but more importantly he wants to take points off Piastri which helps Max.
And the closer he gets to Piastri as the season ends the less McLaren can afford to take a chance and prioritize Piastri even though he's still leading because a single race could now swing the pendulum completely.