Fallout has trained us in recent games that junk is junk and sell it out or it slows you down. This of course was an exception in Fallout 3 when you got the Rock-It Launcher and used it as ammo.
However, Fallout 4 has introduced the crafting mechanism which allows you to modify your weapons, armor and even create entire towns. These changes and created items require materials which you gather from breaking down junk materials and even items in your environment (cars, houses, trees.)
Scrapable Items (Not a complete list)
Acid – Insect parts like bloatflies, bloodsuckers. Batteries.
Adhesive – Wonderglue and duct tape.
Aluminium – Aluminium cans. Coolant caps. TV Dinner trays.
Antiseptics – Abraxo Cleaners. Turpentine.
Asbestos – Abraxo Cleaners. Rat poison. Oven mitts.
Ballistic Fiber – Only available in shipments at merchants.
Bone – Skulls. Teeth.
Ceremic – Dinner plates, cups and mugs. Toilets.
Circuitry – Machine gun turrets. Some tech items like phones.
Cloth – Clothing of dead raiders. Sofas and cloth chairs. Rugs.
Concrete – Cinder blocks. Ruined Houses.
Copper – Various metal items. Broken light bulbs. Lamp Posts.
Cork – Antique globe. Baseballs.
Crystal – Crystal Liquor Decanter.
Fertilizer – Bag of Fertilizer.
Fiberglass – Weapons. Rat poison. Shipment of Fiberglass item. Abraxo Cleaner. Telephones.
Fiberoptics – Biometric scanners.
Gears – Toasters. Other mechanical objects.
Glass – Empty Nuka Cola bottles. Other drinks.
Gold – Gold pocket watches and other jewelry.
Lead – Pencils. Batteries.
Leather – dead clothing of raiders.
Nuclear material – Fusion cores. Alarm clocks. Biometric scanners. Some board games.
Oil – Lighters. Gas canisters.
Plastic – Plastic silverware.
Rubber – Rubber tires.
Screws – Toy car. Toasters. Antique globe. Other mechanical objects.
Silver – Silverware. Silver jewelry. Turrets.
Springs – Lighters. Toasters. Other mechanical objects.
Steel – Ruined cars and houses. Furniture. Toasters. Toolboxes. Lamp posts. Rubble. Tire irons.
Wood – Trees. Home furniture. Picket fences.
So with adding crafting options they had to add places that you could make these items/changes to your objects.
Chemistry station: For crafting drugs, medicine, grenades, mines, Syringer ammo, and Cutting Fluid
Armor workbench: For scrapping, repairing, and modifying Armor and clothing
Power armor station: For repairing and modifying Power armor
Workshop: For scrapping and building buildings, furniture, and various equipment for settlements.
Cooking Station: For preparing scavanged edible items.
Weapons Workbench: For scrapping and building weapon mods using parts from a variety of weapons.