I thought it was like why Jack had to get his father's shoes -- it has to do with having "something" belonging to or relating to the original 815ers on the plane as much as possible. Someone told Hurley to bring something of Charlie's (or something that reminded him of Charlie -- he didn't have Charlie's original guitar case, I don't think, did he?) Now, who that someone was is up for grabs... either Ben or Charlie himself? Maybe the lawyer said "Ben told me to tell you that you have to bring this guitar case" when he handled the paperwork to spring Hurley from jail...
I agree that they just have to show us what happened to all these people in the interim before getting onto the plane -- they have a lot of ground to cover. Kate putting Aaron somewhere, Ben getting beat up, Sayid getting captured and "deported," Hurley getting out of jail.