A "billet" is a round-cornered square-section long form produced by rolling.
Wikipedia mention of product form, including billets [external link]
Its sole application is further processing into engineering components - typically by forging.
It is therefore a crude form. The rounded corners make the rolls cheaper and more robust, plus when forged it behaves well, as the rounded corners have no tendency to fold-over and produce laps in the product.
(R. Smith, 31Dec2017)