Some benefits of omniscient third person are the emphasis on character and how the reader identifies with the author. Because the author is not a part of the story, as with first person, the writer may focus solely on the characters, plot, and surroundings. In contrast to first person, third person allows the reader and writer to not be forced to identify with the protagonist of the story. Instead, there is distance between the characters and the reader that allows for the author to use his own voice, which the reader can then identify with and immerse herself within to fully enjoy the story.
Omniscience is the ability to know everything at any time: it is god-like in scope. This type of narrative allows the author to show the reader everything happening around a character and in each character's head to grasp the full picture. Everything shown to the reader is the absolute truth, whether the characters know it or not. Also, when it comes time to pull forth that knowledge, point of view is crucial. One character may look away and miss an expression on another's face, but the reader will see it and hear that person's thoughts as the point of view shifts. No tropes necessary.
An author's writing suffers from narrative mistakes. Amateur authors will often "tell" rather than "show." In other words, they rely on omniscience to describe everything rather than let the characters do it themselves. By doing so, the author loses the reader's interest in the story. At the extreme end of this is the "infodump": the author gives the reader all the information she requires in a lump rather than playing it out with the characters through the story. Readers are also merely observers. First person allows them to participate and make mistakes while third person removes them entirely from the story.
In the same vein as the "infodump" is the author's susceptibility to digression. Digression happens when the writer, whether to cause tension or because of ego, decides to discuss something related to the character or situation but not the storyline. The diversion does nothing to advance the plot. The final con is the double-edged sword of omniscience itself. An author may employ this type of narrative for its own sake without giving information to the reader. Some readers may discover that knowing absolutely everything makes the story dull.