Great question. And one for which I didn’t have a ready answer. But thanks to one of my students we found out at least one good reason. You see it would seem to make sense that a digital camera should not work without a memory card. Who wants to be shooting all day just to find out that there was no memory card to record the images inserted into the camera. Some cameras have a warning in the menus that you can switch on to alert you to that there is no memory card present. But why not just make it necessary to insert a card before shoot?
Here’s the answer and I really should have remembered it. Some photographers, especially those working in a studio with their clients reviewing their work as they shoot, don’t use memory cards but have the camera tethered to the computer through the USB cable so their images are (a) saved on the computer’s hard drive and (b) are immediately visible on the computer screen.
Of course! And, I saw this in one of the online You Tube photography videos. Thanks to my astute student for reminding me and offering this explanation to the class.