If you've ever tried riding somewhere with a heavy backpack, you can appreciate the practicality of having a rear rack to carry stuff.
When I built my bike years ago, I used cheap second-hand racks made of steel, and all of them broke under regular commuting use. So I went about finding a proper replacement, and stumbled upon the Axiom Journey rear rack.
The rack itself is made of welded aluminum tubing. It's surprisingly light for how strong it is, as it's rated for 150 pounds and only weighs 1.5 pounds! Believe me when I say this thing is beefy. The most I've carried on it is about 80 pounds.... one of my kids rode sitting on it with no problems. I'll just say my rear wheel would probably blow spokes before the rack breaks.