because in the french you want to undermine white's center. you will push your pawn on f6 too and try to isolate the e5 pawn, but be carfull when you play f6.
Why 3...c5 against the French Advance
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What else would you do? It's not so easy to find another usefull move in that particular position.
Why is 3...c5 the overwhelmingingly favorite move against the French Advance variation (1.e4 e6 2.d4 d5 3.e5)? Yesterday when looking up some stats on 365chess.com 3...c5 gets played something like 97% of the time.