It just makes it clearer that he wants it to run at 100FPS: (1000ms / desired FPS)

The usefulness is more obvious when you want 60FPS, which is not easily represented without rounding.

It was probably for readability, i.e. "100 times per second".

Similarly, people will write (10 * 60 * 60) if the value is seconds and the programmer intends to indicate 10 hours.

It's a good way to remind you the figure is in ms, and easily manipulate the interval as a fraction of a second.

I personal do this for legibility and ease of comprehension. Kinda loses its value when dealing with such nicely rounded numbers but, as an example, 1000ms / 24 is easier or me to comprehend as 24 ticks per second than 41.6666ms.

Probably just to emphasize that clock will be called 100 times per second (which will result in 100 fps).

100 frames per second. He probably changed that during development.