As an Awoken prepares to leave its physical being to become a custodian of the Wildwood, they harden their bark for ensured protection. I've observed that they also put down more roots with each step towards death, strengthening the entire Wildwood in the process.
Heartstone is effective on units that typically occupy the front line of a floor. Front line units are the ones that take most of the damage, leaving the role of dealing damage to the units safely tucked behind them. Units with the Revenge affix also want more survivability, allowing them to benefit their Revenge ability as much as possible.