Diplomacy: Good NPC's will be a LOT easier to talk to to get information (even some quest realted) and thank you more (rewards). Intimidation/bluff: Evil NPC's will spill a lot of information before you kill them and possibly loose morale quicker in battle. Being skilled in diplomacy might even help you avoid confrontation (In RPG's, that is...)
I think all of these skills will get you better prices in shops as well.
However they're not essential, but adds variety to the storyline. So if you're a "power-gamer" forget about it.
Other factors are: High intelligence and wisdom scores will also give you more dialogue options. High charisma will make people approach you in a less hostile manner and reduce prices.
Clear examples: None that I can remember - I might just have made my entire reply redundant here
