An open house when your property is first placed on the market can be very important, but not for the reasons most homeowners think. Just like with advertising, most visitors to open houses rarely buy the house they come to look at. They may not even know the price of your home when they stop by to visit – they probably just followed an "Open House" sign to your door.
An open house performs a similar function to the neighborhood announcements – it lets all of your neighbors know that your house is for sale, and it practically invites them to come "take a look." Being generally nosy, a lot of your neighbors will take advantage of the invitation.
And they may tell their friends about your house, creating more "word of mouth" advertising.
Of course, there are other reasons for holding open houses, too. Listing agents who "farm" a particular neighborhood use them as an opportunity to meet with other local homeowners who will someday be selling their home. Your agent may hope to list their homes in the future.
Open houses held after your home has been on the market awhile do not usually serve a useful purpose in selling your home. Most of the neighbors already know your house is for sale and open house visitors rarely buy the homes they visit.
However, if you really want more open houses, your listing agent may allow other agents to hold it open. Open houses attract prospective homebuyers and agents hope to convince some of those homebuyers to become their clients.