>>579736
Entertainment. Academic study. Reference material. Preserving knowledge for future generations.
Even if you never build, say, an MG-34 from scratch, having the technical data packet allows you a better understanding of the manufacturing processes used and design choices made in producing that particular firearm.
Furthermore, we collect blueprints for the same reason a zoologist or botanist collects gene sequences. As long as the pattern is in the archive, a gun can never really go 'extinct'.
>>580024
weaponeer.net is a good source for low-hanging fruit
Also, see >>577479