Not every character who strolls on scene in your story deserves a name, only those who contribute to the plot in some way. Consider that you encounter numerous nameless people every day, whether on your daily commute or even at work. In fiction, these often include incidental characters such as waiters, people in the elevator, some coworkers, etc. Describing their appearance for the sake of imagery, however, makes them seem real. This is when unusual or outstanding characteristics help them stand out and add to the story, even if they have no plot significance.