The bolt clip (that piece of metal that holds the bolt on the rail) was developed because castle defenders in medieval times had a minor issue with their bolts falling out of their crossbows when they leaned over the battlements to shoot at the people on the ground. I would imagine it was a tad demoralizing to see a whole line of crossbowmen loose their bolts over the side of the walls when the command "AIM" got shouted.
Wyvern