The list price is part of the seller's marketing strategy. Priced high, hoping for a sucker or a lower but reasonable offer. Priced low, hoping the market writes multiple offers trying to push the price up. Priced in the middle hoping to settle close to this.
The sale price is what the buyer and seller finally settle on. It could be higher, lower or equal to the list price.
Jennifer nailed it. Realtors will use this information to give a buyer/seller a gauge for where the market is at. If, on average, homes are selling very close to the listing price you can likely infer that there is a strong demand, which is what we're seeing in many areas of Seattle.