1. She's the love of Buffy's life and gives her the strength to go on.
2. As the rest of the Scoobs become more capable she's the everyman/woman, she represents the ordinary over the super powered.
3. She's everyone's little sister/daughter
4. It's wonderful to watch her grow, like the early Scoobs between 1-3
5. Michelle gets really hot/legal by s7.
That'll put marzipan in your pie plate, bingo!