well, as sarge said, some never have the chance to meet their star in person. another reason to buy an autograph is you once had one and it got lost/destroyed.
but as sarge said as well, to me it is much more important under which circumstances you got it. every autograph i have (james stewart or robert beltran or alice kriege or ....) has its own story. and to me it is the story that counts and the autograph is a reminder of it
