Usually, phantoms are just an annoyance that get disabled with a gamerule command.

But, if there was a reason to deal with them, people might actually engage with them, so...

• Every 3 days, phantoms will get larger and more dangerous. But, if you can endure them, you'll get a reward the next time you sleep.
• A permanent absorption buff, until you lose your extra hearts, The level increases with every 3 days, starting once the phantoms begin to spawn, you will get level 1 absorption.
• Both phantom buffs and the absorption buff cap at 12 days.