There are a number of situations in which it can be advantageous to create a visualization to represent a single number:
To communicate with less numerate viewers/readers;
Infographics and dashboards commonly use one important number;
To attract the attention of distracted or busy viewers/readers;
To add some humanity or “color”, to create an emotional connection;
Or to increase the redundancy of the presentation (see Improve the Quality of Data Visualizations Using Redundancy).
To a great extent, my favorite is the first. There are good cases for many of the others—primarily the shock value of the uncountable pictogram—but typically, the best visualization is simple.