DDT,
Let me climb up on my soapbox . . .
The main use of core aerators is to break through compaction on top of the soil. The implement cuts and removes a core (1/2" to 2" diameter, 1" to 4" long, depending on the unit) of material (grass, thatch, soil, etc.) from the ground, which then allows the movement of air, water, etc. down into the ground. Basically, a core aerator is a roller with a bunch of open pipes sticking out all over it. The pipes poke through the compaction and pull a plug of material out of the ground.
The REAL question is what causes the compaction . . .
Driving tractors and other heavy equipment over the ground is a good way to compact soil. The soil around a newly built house will usually be pretty well compacted from all the construction equipment. Highway departments use what's called a sheep's foot (a roller, sometimes filled with water, with metal 'feet' sticking up all over it) to compact a road bed before paving.
The main cause in grass yards is from using a mulching mower instead of bagging cut grass.
The problem is that not all of the cut grass breaks down, "to provide nitrogen and other nutrients for a lush, green lawn" - regardless of the claims of sales people. Over time this cut grass mats down, and must be removed with a dethatcher. Some people use a core aerator to cut through the mat, instead of dethatching.
/w3tcompact/icons/smile.gifAs you might guess, this is a pet peeve of mine. I really detest the fact that sales people don't give people "the rest of the story".
Pls pardon the preaching - I hope the rest of the info helped.
Dave Wells