Most sellers want two things: the highest possible price and the shortest possible time between listing and closing. Knowing the current value of your home provides you a reference point to help you in reviewing offers.
There are several ways to guide you to an accurate value such as running comparable comps for properties sold within the past 90 days, square footage within 20% of your current home, similar number of bedrooms and bathrooms, and homes within a mile of your current home.
Real Estate Agents have access to the Multiple Listing Service and can prepare a Comparative Market Analysis for you using comps, tax data, homes that are currently listed or recently sold in your area that are comparable to your current home. With this information your Agent can help you calculate an estimate on the current value of your home.