If it’s necessary then how did my last tank make it 20 years without one?

What’s an expansion tank? I set it up like the old one was set up. All I did was disconnect the old one and reconnect the new one to the same pipes coming out of the wall.

I worked at a 3rd party Apple retailer (they had a legacy contract from the 90s that only expired about 5-10 years ago) and they bought the cheapest RAM they could find to upgrade the Macs. They made hand over fist on RAM upgrades and still came in under what Apple charged for the same upgrade.