Pearl Harbor was almost an afterthought in the original Japanese planning. It wasn't until October 1940 that it was even considered as a potential target.
Their major reason as Wild says was to cripple the US Pacific Fleet (which up until mid 1940 had been based on the US West coast not Pearl thereby rendering it immune to such a Japanese attack) now that it was just within striking distance of the Kido Butai (the Japanese fast carrier force).