For one thing, it’s much easier to measure spans of time when you have an integer frame rate. For example, 1 hour at 30fps is exactly 108,000 frames, but at 29.97 it’s only 107,892 frames. Since frame numbers must all have an integer time code, “drop-frame” time code is used, where each second has a variable number of frames so that by the end of each measured hour the total elapsed time syncs up with the time code, i.e. “01:00:00;00” falls after exactly one hour has passed. This is of course crucial when scheduling programs, advertisements, and so on. It’s a confusing mess and historically has caused all kinds of headaches for the TV industry over the years.