Trademarks are very useful in conjunction with inventions, whether patentable or not. A clever TM can be used with even an unpatentable invention to provide it with a unique aspect in the marketplace so that purchasers will tend to buy the trademarked product over a generic one. For example, consider the Crock Pot slow cooker and the Hula Hoop exercise device. These trademarks helped make both of these unpatentable products successful. In short, a trademark provides brand name recognition to the product and a patent provides a tool to enforce a monopoly on its utilitarian function. Since trademark rights can be kept forever (so long as the TM continues to be used), a TM can be a powerful means of effectively extending a patent monopoly.
