Writing stories gives a background for the community to use, something they can build off of- in RL conversation, you use what you know of someone to find something to talk about. Ingame, if you've read about a character you have a starting point, a history that you can ask a question about, something to get talking started... after that, the RP just flows like a river.
I write because I like to, and because people like my writing. If it makes their day better, that's good enough for me. If you can write, please do, it gives us a nice break from the rough forum politics, something we can learn from, something to help structure how we see this little place called Sirius. By no means a requirement for anything, but if you do something out of the ordinary and someone asks you about it, its a lot easier to point them to a link on the forums than it is to explain the synopsis of your idea to everyone who asks about it.
Also, a story really gets into the head of a character- if you have an idea and don't know where to go, the story will give others a place to start, something to work off. Characters and players can interact in ways you could never do ingame, and after awhile you can figure out what your character will do in any situation without second thought... quite useful in the rapid pace of ingame action.
Its something fun, its something helpful- if you know what you need to and you can write, do it. If you don't know the finer points of Sirius, either ask or browse stories. Not much more to it. Its something that I wish more people did, but if you can't write then you don't have to. No biggy, it isn't your field. Oh well.