Fortify can be used as an alternative finisher option to Pink Bloon in aggro decks by directly adding more health to an existing zero-delay Bloon, allowing it to deal more damage when it hits the enemy Hero.
Bloons with Double Attack also gain double the damage from Fortify's bonus health, for essentially +100 Hero damage. Fortify can also be used in combination with Quick Ready on Double Bloons.
Triple Threat Bloon gains the most benefit from Fortify alone. Be careful when adding Fortify before it leaks, otherwise Return to Sender will gain value.