Number 10 Downing Street, commonly known as Number 10, is one of the more famous addresses in the world so it's no surprise to see it featured on a postcard. It has been the official residence for the British Prime Minister (as the First Lord of the Treasury) since 1732. The front door hasn't changed in appearance since 1772 though the original oak has been replaced by blast-proof steel since the IRA activities during the 1990s. If you're interested in its history, the Wikipedia link is very interesting.
The first card dates from 1980, the second is one of the postcards representing the current issue of 12 First Class stamps. The first was used as a party invitation, the second will be used as part of a project I have just discovered from The PenPal Project, via STUFFellaneous.
A school in Florida is collecting postcards from all the USA and around the world to celebrate their diversity/international day in the spring. Maybe you'd like to send a card too, to:
8443 Sand Point Drive West
Jacksonville, FL 32244