"Happy new year!"
Translation:Bonne année !
It is to match the quality of the vowel sound. ( If I'm not mistaken this accent is only used for this vowel).
è (accent grave) is for softer sounds, é (accent acute) is for harder sounds. Just try to think of the G-sound from grave which is soft, and the A from acute which sounds harder when pronounced in French.
For Dutch natives there's a different trick I learned in school, but it doesn't translate to English.