A well designed website always has the user in mind. It will give visitors the information they need and clearly show them how to find out more and / or get in contact with you resulting in lots of sales enquiries.
A poorly designed site does just the opposite. Without finding the information they needed your visitors left and looked elsewhere resulting in little or no enquiries.
This infographic from LogoSnap gives you 7 examples of poor design that could be costing you money.