Indeed - I have always thought (though I don't know if I was told this or just made it up) that IV was used, in general, over IIII because the latter could easily be mistaken for III, but as clock numerals are on a circle, dividing it into four equal quadrants, having a numeral of any sort gives very little additional information over a simple dots, pips or tick marks.

So why use a numeral at all? Well, there's always those people who will think that it is obviously wrong not to have numbers on a measuring instrument. Also, in early modern times, clocks were expensive items and expected to be ornate, especially as they were not all that good at keeping time.