Here's why:
* Gender in French: Many nouns in French have a gender, either masculine or feminine. This is often not logical and must be learned.
* "Avantage" ends in "-age": Nouns ending in "-age" are generally masculine, with a few exceptions.
So, when using "avantage" in French, you would use the masculine definite article "le" or "l'" before it, depending on the following word.