There was a time when chain guides, bash rings, even front derailleurs were commonly mounted behind the drive side bottom bracket. So, many of them were designed 2.5 millimeters too wide in order to accommodate the width of the extra accessories. Today, they're much less common, so most bottom brackets use one spacer to bridge the gap. And you'll have three of them if your frame has a 68-millimeter shell instead of 73.
But most stock spacers are plastic. Whenever you're tightening a bolt, it's always nice to tighten against actual metal, so this bike hack suggests just that. U.S. manufacturer of bearings, derailleur hangers, and all things forgotten and un-glamorous makes an alloy spacer to do just that. And if you've got chainring clearance or chainline issues, you can break that spacer up into one 0.5-millimeter and two 1-millimeter spacers to slide the whole show over in precise increments.