Tin cans are sealed metal containers used to hold and preserve perishable food, such as Pork n' Beans. Their airtight seal allows food to keep for years, though all Tin cans found are open, empty, and unlabeled. In-game, they are typically found either in places where food is or was once sold, such as grocery stores and gas stations, or alongside camping grounds and hideouts.
Used as ammunition for the Rock-It Launcher, tin cans are especially useful due to their relatively low weight, expendability, and prevalence.
They are an essential component in the creation of Nuka-grenades.
Also in Dead Money, after you get the Junk Rounds perk you can use tin cans, scrap metal, and the various cases and hulls you find to make "junk rounds." This crafting requires five tin cans, one piece of scrap metal, and a case or hull for the ammunition you would like to make.
Similarly if you east from the Allied Technologies Offices and enter the abandoned warehouse there are at least 18 on the shelves in the back of the room near the large storage containers. (The abandoned warehouse is used as the meeting place for the Van Graffs and Caesar's Legion during the quest Birds of a Feather).
There are 10 tin cans located at the Mesquite Mountains camp site scattered around the fire pit and the picnic tables along with several bent tin cans.
There are 8 tin cans located at the Tribal village they are located around the campfire and may be covered by the bodies of the cazadores that inhabit the area.
In Vault 3, home of the Fiends. there is a total of 42 tin cans scattered throughout. 26 of them are scattered about in the Recreation Area, with almost all in the halls. In the Living Quarters there are 16 tin cans, with, once again, most located in the hallways.
Near Powder Ganger camp west there is a destroyed truck backed into a large radioactive pit, at the end of this truck there are 4 cans along with several milk bottles and bent tin cans.
In the Bison Steve Hotel there are 9 tin cans inside the locked storage room to the right of the main entrance. 4 more can be found in the hotel's cafeteria where several of the convicts are located.
At the NCRCF there are 5 cans behind cell block B among the radioactive pools and other garbage. 2 more can be found in the northeast corner guard tower. 2 more can be found in the western guard tower closest to the visitors center.
6 cans may be found at the Nipton Road reststop with two in the garage separate from the main building and 4 on the front porch of the store.
14 cans may be found in the fenced in area of the Gibson scrap yard, 9 found on the west side, stacked in a nice little pyramid, two near some locked metal boxes on some shelves, and 3 near a large, overturned dumpster in the northeast corner of the area.
There are a few tin cans in the houses in Nelson, though they have to be stolen.