The Sisters' Powers:
The sisters are given powers of the Greek Gods - Phoebe of love, Paige of War and Piper of Earth. Note that Goddess of Earth in mythology would be Gaia, who was Mother Earth - and gave birth to both the Titans and the Gods. Paige and Phoebe both try to rule and be adored rather than stop the Titans when they first receive their powers - but it's Piper's grounded influence that stops them. Perhaps Leo gave Piper the powers of Gaia because he knew she'd be the grounding influence that would stop Paige and Phoebe going mad with power.