Most tires are filled with compressed air, which when dry consists of about 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases by volume.
Water vapor (humidity) can make up as much as 5 percent of the volume of air under worst-case conditions.
Filling your tires with nitrogen mainly does two things: it eliminates moisture, and it replaces skinny oxygen molecules with fat nitrogen molecules, reducing the rate at which compressed gas diffuses through porous tire walls. Removing moisture is a good thing as the presence of moisture means that compressed air does not follow the ideal gas law as the temp passes through 100 degrees.
That means, theoretically at least, that a tire filled with nitrogen retains optimal pressure longer and provides more predictable behaviour when worked hard.