Essentially: The mere symbolism of it. People ascribe a bit more meaning to such 'round' numbers, and since you are in a market with other people that do the sa

Essentially: The mere symbolism of it. People ascribe a bit more meaning to such 'round' numbers, and since you are in a market with other people that do the same, it actually starts to make sense to see them as important markers at some point. (That's been my personal explanation for it, anyway.)