I believe most people join in a guild to enjoy the companionship of other players. Another reason might be for help in completting missions. If you've already got a group of friends that you regularly play with, then you might not want or need a guild. However, here are some other things that some guilds can add to your gaming experience.
- live voice chat
- mentors or tutors tohelp you increase your knowledge of and skill in the game
- guild vs. guild play in Lockdown
- an additional website for sharing in forums and posting pictures and screenshots
- a guildhall for decorating and hanging out in
Remember, it's possible to be in a guild and still be lonely. Greeting and socializing in haven with a range of other players regardless of guild will greatly increase your enjoyment of the game, although you should avoid random friend requests and be leary of hit and run invitations to join any guild.
After being in the game for awhile, you may decide that you don't need a guild after all, or that it's about time you joined one. But either way, know why you are joining and keep realistic expecations about what you will get out of and put into your guild experience.
My favorite part of belonging to a guild is the humorous banter that fills guild chat. Besides that, guild runs, weird guild traditions, and warm welcomes for new guildmates are also part of the desired experience.