Magnetic fields, such as you have in a motor, can make life difficult. Whilst they're left alone they don't trouble you, but trying to switch them off can result in a swift kick. The magnetic field stores energy. If you try to collapse the field this energy has to go somewhere. The result is a large voltage across the device - the "back EMF". The swifter you change the current, the bigger this voltage gets. To safe-guard the electronic components driving such a device it is wise to add "clamping diodes" to the output pins. These go "the wrong way round", so that under normal conditions no current passes through them. But in a back EMF condition with the voltage going beyond either of the supply rails, current will flow through either of the diodes, and not the transistors in the driver chip. If you use the diodeless L293 version you should remember to add these diodes externally - or else!