Core aerating is arguably the most beneficial thing you can do for your lawn.  Aeration pulls up to 4" plugs from your lawn.  The plugs then decompose back into the soil.  The holes created then allow nutrients and water to reach critical root areas of your grass plants thickening up your turf. 

Aerating in the spring or fall has many benefits:

Breaks up thatch

increases root depth

improves soil structure

encourages new growth

opens up roots for fertilizer, oxygen, & water

un-compacts soil