Why were beer cans tossed away just anywhere? "Well, DUH!", you may think, "because they were trash!"  True enough, but many breweries before prohibition and even immediately after, required you to return their empty bottles.  One of the major selling points of cans when they first came out in 1935 was that there were no empties to return. It also cut brewery expenses because they didn't have to have bottle washing machinery, a carpenters shop to repair the wooden crates that carried the bottles to the merchants and back, or replace broken bottles.  An early ad promoting canned beer showed a fisherman tossing his empty can into the lake to dispose of it!

Can companies took advantage of this opportunity and advertised the benefits to the consumer of this new kind of packaging for beer.  Below is a typical example, the side panel of a late 1940s Blatz beer can.
 

The side panel reads.. Requires little space Stacks well in refrigerator Safe and sanitary Drink from it if you wish Light cannot penetrate Cools quickly No deposit required No empties to return Call for Blatz - Served at all fine food establishments, taverns, hotels, and clubs.

 A lot of good cans have been found by people doing work in older houses or other buildings.  Often workmen would drink a beer on lunch break and stick the can in a wall or crawlspace when they were finished. Hey, the old cans used to say, "when empty, throw it away"!  These indoor cans turn up now in great shape and can be very valuable. Of course, common cans also turn up in old buildings.  Those construction workers 60 years ago didn't just drink beers that would be rare decades later!  (Alas)  But if you have cans that you found and you want to sell,  please email me.  If I can't take them I will happily put you in contact with several friends and fellow collectors who I know buy cans.  I am also happy to answer questions about any can you find even if you don't want to sell it!

Of course, a lot of later cans had "Do Not Litter" messages.  Here is one of my favorites, from a 1950s Adler Brau Appleton Beer. This was on the lid...

Judging from the rust on this can, the original consumer didn't follow the suggestion on the lid!

Of course, once they were emptied, cans were also turned into other useful objects: lamps, seed spreaders, etc. My November 2010 COM has more details.