The first episode of any television series is important. For completely new shows, it can determine whether a studio will buy an initial season. While the risks are less for a spinoff of a popular show with a built-in audience, especially a flagship show like Star Trek, there are still important tasks to accomplish. The pilot needs to give clear links to the earlier show, usually including cameo guest appearances from an actor on the earlier series. A pilot episode needs to set the scene and establish setting, and it has to introduce and characterize the major characters.
As the pilot episode for DS9, Emissary establishes a number of major themes and story arcs. We also see the important links to TNG, most importantly The Best of Both Worlds, and Encounter at Farpoint. Perhaps most importantly, Emissary introduces the major characters, allowing the audience to identify with them. While it must introduce the main cast, there is a clear focus on the lead role, in this case, Commander Benjamin Sisko.