I'm unclear if this is something that should be done only when a lawn has problems, or as part of normal maintenance every X years? I guess you could even do it every spring but I imagine that's overkill and puts the lawn out of use for a few weeks just as you want to start enjoying it!
Well it all depends on what kind of lawn you want and the effort you want to put into it. If your ambition is to have Perfect grass: perfectly flat, no weeds, no dead spots then over seed twice a year, every year.
This keeps the turf thick, helps out compete weeds and doesn't take very long to do.
If you don't care so much then just over seed in the fall in the bare or patchy spots and then top dress with compost or organic matter.
You don't overseed the lawn, you let it grow long (say 6 inches), then cut it back drastically (say 3 inches), but not so drastically that it will kill it off (say 1.5 inch), because if you do that the grass can evaporate from the surface a lot quicker than the grass can take it in. It will fill in the gaps that are in the grass, and you won't need to do anything to fill in the gaps.
I experimented where I used to park my truck, even though it's coming back some I noticed it come back considerably when I let it grow to the top of my shoes, then cut it back to 2.5 inches, and it "suckered up" very thick all over the lawn, and looks incredibly healthy now. The concept works for trees, because if you remove a lot of growing points on the tree, because you took away a lot of the growing points for the tree, so I thought why not with grass.