[jurip]

Sandi Toksvig made the point in a QI episode that if you use IIII, you get a pleasing symmetry with the number of Vs and Xs on a clock face. Four numbers with only Is, four numbers with a V and four numbers with a X.

I'd be surprised if that was the reason, but it's kind of neat.

I II III IIII

V VI VII VIII

IX X XI XII

[Amorymeltzer]

This is exactly where I first learned about this style; I looked it up and loved it.