The prefixes prae- and ante- both have the same meaning of 'before' in place or time. Why is the existence of both words necessary?
There are differences between the two words. Lewis and Short give a detailed account of both prae and ante. There are situations where they are interchangeable, but they are not synonymous everywhere.
For details, see the two dictionary entries. Here are some points to highlight the difference:
- Both ante and prae mean "before" in a spatial sense, but ante is used for objects at rest and prae for objects in motion.
- Prae has the sense "because", ante does not.
- When it comes to time, ante seems to be far more common for "before". (I get the impression that prae isn't possible at all, apart from giving that sense in compounds.)
When you look at a dictionary that gives a couple of English words for a Latin word as translation options, they can look identical. When you look at a dictionary that gives detailed explanations, much more nuance emerges.
So, the two prepositions are different. Used as prefixes they are much closer to each other than as prepositions. But even if it was not necessary to have both, they can still coexist; synonyms are not that rare. The meanings of the two words have certainly evolved through time, and that might explain how we ended up having these two very similar prepositions. Perhaps ante was more "against" and prae more "before". I would say that it is not necessary to have both ante and prae, but we still do, and it does allow expressing some things more accurately.