From Reuters:
People who accumulate fat around the abdomen and stomach are more likely to die of heart disease and other causes than bottom-heavy people.
People with fat in their thighs and backsides may live longer because the fat traps harmful fatty particles and actively secretes helpful compounds.
Fat on the bottom and thighs appears to store excess fatty acids. Pear-shaped people also appear to have lower levels of compounds called inflammatory cytokines.
Leg fat may also be better at producing hormones such as leptin, which are made by fat and affect appetite and metabolism.
References:
Why those fat thighs may help you live longer. Reuters, 2010.
Image source: Williams pear, 1822 printing from the Horticultural Society of London. Wikipedia, public domain.