It makes no difference? This isn't right. Holbein was very careful what he put in his paintings.
It's not to make him seem shorter either. The painting is larger than real life so Henry does not seem short.
The correct answer is that it makes him seem above the viewer. Henry was the most powerful man in England. He was socially 'above' everyone else. This step also makes him physically above everyone as well. He is looking down on his people.