In the US, the postal service has been pushing collection of "uncancelled" sheets, and it apparently is popular enough that they introduce "lots" (I have no idea of the number) of commemorative designs. Most collectors buy full sheets and salt them away somewhere in the hope of making a profit some day. (It's a little difficult to display full sheets?) A few collect "quarter sheets." It apparently is profitable enough for the USPS to get a significant input to their operating expenses.
Collecting of cancelled stamps has all but died here, since in most cases the stamps aren't cancelled and have no indication of where or when they were mailed. While you might make a collection of stamps from different countries, making a collection of "letters from different cities" in the US would be almost impossible, since they're not marked to indicate a place of origin.
Occasional "issues" will allow you to buy a commemorative with a "hand cancellation" showing the post office of origin and the date, but there's some difficulty in getting those (which are intended only as "collectibles") unless you happen to be in the right place "on the day," or subscribe to info bulletins so you know when to order them.