They're inflation-protected, and unlike TIPS, they're also deflation-protected. TIPS can dip into negative interest rates, while I-bonds can't go below 0. And since they're variable-duration bonds, they aren't as affected by interest rates swings like regular bonds.
If the bond is used for qualified education expenses for yourself, spouse, or dependent, it’s tax exempt. If you have kids or want to take any classes yourself, it's a nice bonus.