On a blade switch, each tab has a corresponding eyelet - a little round opening lined with metal - that is actually on the wafer board. (You can see them in the image below.) When you pass a wire through one and solder to it, it is the same as solder to the corresponding tab.
I've never, ever used them. The only reasons I could see for someone specifically instructing you to use the eyelets would be:
1) They think it's easier to use an eyelet when two wires need to be connected to the same tab - you can solder one to the tab, and one to the eyelet. (IMO, it is easier to just solder them both to the tab.)
2) They think it creates less clutter to use eyelets when the tabs that wires needs to connect to are on the far side of the switch. It keeps you from having to pass them across/over the terminals.
3) It's "vintage correct" in some way.