In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

The LM339s are also controlled by a 50K pot that nicely allows you to adjust the brightness of the display.  By getting this taken care of at the hardware front end of the amplifications stages with a comparitor, you don't have to rebuild your circuitry if it is too dim or pulling a potentially damaging level of power - just tweak the pot.

There are two ways to control brightness - in software by controlling the duty cycle AND in hardware by a scheme such as this.  Avoiding having to fine tune brightness in software gives you more programing space to do other things.  It also allows you to start out with a dim display and protect it from potential damage due to a miscalculation in software.

It is a very intelligent design.

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