Unit Commander P-59, along with its subordinate, P-60, were the pair of droidekas that Nute Gunray, the Trade Federation Viceroy, deployed. Their mission was to prevent Qui-Gon Jinn and Obi-Wan Kenobi from gaining access to the Saak'ak's bridge. The intense barrage from the droidekas compelled the two Jedi Knights to escape through a ventilation shaft.
According to Star Wars Episode I: I Am a Droid, the designation P-59 refers to a class of destroyer droid, and not an individual unit's specific identifier.