Why use [img][/img] for pictures

[Ruark]

Seems like almost all online forums use the [img]<imageURL>[/img] method for putting pictures into posts. It seems like it would be a LOT simpler to let people upload it off their hard drives using a browse button, but you almost never see a forum using it. I'm wondering what the specific reasons are for this. I know server storage space can be a factor. What about security, or other reasons?