The term suprasegmental refers to those properties of an utterance which aren't properties of any single segment. It would have been logically possible for every syllable to have exactly the same loudness, pitch, and so on. But human languages have ways to make some syllables more prominent than others. Prominence is relative to the surrounding syllables, not absolute. Boundary marking In normal speech, words and phrases simply don't have little pauses between them.
Prominence can help indicate where the boundaries are, making life easier for the listener.
French usually gives prominence to the syllable at the end of a word or phrase. There seems to be a bias for English listeners to interpret a stressed syllable as the beginning of a new word. Children will delete unstressed initial syllables more often than unstressed final syllables. In many languages, changing which syllable is stressed can change the meaning of a word. As well, there are major differences in the articulation of vowels between stressed and unstressed syllables.