Accordingly to The UCAR (University Corporation for Atmospheric Research), software design is the process of defining methods, functions objects and the structure and interaction of the code in order to have fully functional programs and satisfaction of the users.
Here’s a really clear video taken from TED x conferences with Jackson Stone showing the importance of S.D.
Software design is all about thinking in the users themselves and giving them the opportunity to have a full experience when approaching to technology by letting them have a nice experience.