Aerating removes small slices of soil and turf and the remaining soil then has room to expand and valuable oxygen and water has room to be introduced. It is not needed very often unless your lawn is very compact. If there is a lot of foot traffic or vehicles on the area, the compact soils are difficult for new roots to grow in, and there will be less nutrient and oxygen flow through the soil. Plus, water 
will not soak in well, all of this making it a less desirable growing area. If you want to aerate the soil, you can use a hand-held aerator for small spaces. But for a normal lawn size, it is in your best interest to rent a mechanical aerator for best lawn maintenance results.