So they can do a once off capital return to shareholders without increasing dividends.
for example-
Iron ore and oil prices may be strong for a few years resulting in BHP having large amounts of cash which they wish to return to share holders. these strong cash flows may not last forever so the company would not wish to raise dividends or they would just have to reduce them in the future which means shareholders would be confused as to the future direction of dividends.
By buying back shares they are still maintaining the current divy, but also returning the cash in a tax effective way.