Open houses are a great resource for buyers and sellers alike!
For those selling a property, an open house give you the opportunity to have many potential buyers through the home at one time versus many showings. Many agents have a detailed marketing plan for just such events to up interest and drive traffic to your open house. You can be a nosy neighbor and pop in to area opens to get a feel for your competition.
As a buyer, open houses allow you to preview many houses in one afternoon and at your leisure. These events are great if you are just starting out and getting to know a neighborhood.