Guild Halls primarily allow you to do Guild vs Guild (GvG) battles.
They also as a bonus feature allow you to include "have hall" in your "recruiting for guild" message, should you be desperate enough to need such a message.
In addition, you can meet your guildies there and have wild parties without anybody interrupting you with "HEY, anybodi wanna d0nate/l0an me goold for ma arm0r???"
I use my guild hall as a loading area. Not many people, very short load time.
Also, I tend to start my day roaming around my hall. We have Isle of the Dead, the only other one I would have wanted was Wizards, due to it being Ascalon.
