Although the reason to pick a coffee table that is rectangular may be purely a design choice, some practical reasons can come into play. If you use the table to rest a drink or a plate on, you want a surface that is easily accessible from any seat on your couch. This is something that is not easily done with a round or square coffee table, depending of course on the size of your sofa and the rest of your seating arrangement.
Modern rectangular coffee table:
Modern rectangular coffee table: Enitial Lab Monroe Rectangular Coffee Table, Walnut and Black