A lot of grammar references suggest that you hyphenate compound modifiers if they come before the noun but not after the noun. Here's an example:
- The apartment is off campus.
- That is an off-campus apartment.
One of the references that I saw, https://www.quickanddirtytips.com/education/grammar/are-you-using-hyphens-correctly, states this:
Ages are like every other compound modifier that way: you hyphenate them before the noun but not after the noun.
So it says that compound modifiers are not hyphenated when they come after the noun.
I have also read many other English references that say that. Why is it the case? Aside from
it is good and
it is recommended, is there a technical reason or it is a matter of style?