From The New York Times' online Play magazine (you can subscribe to it for free)

At Kona, 415 of the 1,677 competitors in the run portion of the race

wore the socks, said Chris Bohannon of the triathlon gear manufacturer

Zoot. They wore them to increase blood flow, supposedly supplying lower

leg muscles with more oxygen and more evenly distributing muscle

strain. Compression socks were originally used by diabetics, pregnant

women and travelers seeking better circulation and preventing blood

clots.

Studies of runners in New Zealand found mixed results — compression

socks didn’t help middle-distance runners but “there does appear to be

some benefit of wearing these garments on maintenance of leg power,”

Ajmol Ali, a sport and exercise science professor at Massey University

in Auckland and lead researcher on the studies, wrote in an e-mail

message.