What numerals are most common today?
Most people don't understand Roman numerals so movie makers use these numerals to disguise the fact when the movie was actually released. +++ How do you "know" without asking them, that "most" people don't understand Roman numerals (they can always find out!), and why should film-makers want to disguise release dates? Roman numerals have always been common in publishing generally.
MCCLXXIII equals MCCLXXIII in Roman numerals. That is, it is already in Roman numerals. However, if you want to know what MCCLXXIII is in Arabic numerals, the ones most of us use today, here is the answer: It translates to 1,273. M=1000, C=100, L=50, X=10, I=1. Two numerals also used but not found in your example are D=500 and V=5.
Yes the numerals have the same values as in the times of the Romans but today they are calculated differently in the way that the Romans actually did themselves. For example today we would write out 1999 in Roman numerals as MCMXCIX but the Romans probably wrote it out simply as IMM (-1+2000 = 1999)
Because the Latin language is still used today and Roman numerals are in fact Latin numerals. Roman numerals are most often used to represent the year of a date in a more formal way, for example when they show the year in the end credits of a movie, or when showing ordinal numbers, like the third, fourth, fifth of something, etc.