One reason why you might want to overseed your lawn is that your lawn is thin. A thin lawn means more room for unwanted weed or undesireable grasses to make their way into you lawn. By overseeding and existing lawn, you are going dramatically thicken up your lawn making it much more dense, leaving no room for weeds or undesireable grasses such as quack grass to grow.
Another reason you might want to overseed is if you had insect damage in the past that has left you with bare areas. Similar to a thin lawn, these bare are leave lots of room for weeds to grow and rapidly spread. The third reason why you might want to overseed is to introduce new species of grass into your lawn. We can customize to you needs and desires. For example, some new species of grass are much more durable than others and they can tolerate the abuse of high traffic and summer stress much better.