Why do we say that a plane is "en route"?

My primary language being French, I can clearly understand what en route means ("on its way"). I can't see, however, why English-speaking people would use this French expression.

Why is it used?

Solution 1:

On the way can mean a number of things:
- in transit (to): "Flight 105 is on its way to Los Angeles"
- going the right direction: "He set them on their way"
- along/beside the path: "On our way, we saw a lot of wildflowers."

By contrast, en route (in English usage, anyway) only means "in transit (to)." It can be quite useful to use a word or phrase with no ambiguity. Also, from my (admittedly brief) review of Ngrams/Google Books citations, it seems that the phrase was first brought into English in military contexts; logistics is certainly a field where clarity and concision are prized.

Solution 2:

I have studied English for a long time, and one of my pet projects has been to find words that originated in the British Isles before all the invasions began, a couple of thousand years ago.

To date, I have found precisely 0 (zero) words that cannot be traced back to the languages of the various invaders -- we don't even know the name the Beakers called themselves (the name "Beakers" was given to them because they made a lot of clay beakers).

So finding that we use a word from another language is not at all an unusual event -- All of our words come from other languages.

Solution 3:

Most languages have adopted "isms" from other languages, when there are few convenient expressions in the first language for a particular concept. "En route" just happens to be a French expression widely adopted in English. Ditto for "savoir faire," "faux pas," or RSVP (Repondez-vous s'il vous plait.)