YSK: water heaters have an anode rod that prevents the tank from corroding. If you replace it every few years, it will extend the life of your water heater from ~10 years to potentially 25+ years.
from /r/YouShouldKnow
YSK：热水器有一个阳极棒，可以防止水箱被腐蚀。如果你每隔几年就更换一次，就可以将热水器的寿命从~10年延长到可能的25年以上。