Why go
Praha 1 covers much of the historic centre, but it does not tell you whether a place is in Old Town, Josefov or Malá Strana. Learning those quarter names makes route planning easier, especially when a bridge or a castle hill separates nearby-looking sights.
The full story
Prague uses numbered districts alongside older quarter names. The systems overlap, so a Praha number alone is a blunt way to describe the historic centre. Old Town, or Staré Město, sits on the right bank of the Vltava. Josefov, the historic Jewish quarter, lies within it. Across the river, Malá Strana climbs toward Hradčany and Prague Castle. Nové Město, the New Town laid out beside Old Town in the fourteenth century, extends through parts of both Praha 1 and Praha 2. Vyšehrad is farther south in Praha 2. These names give a better sense of the ground than a district number: a visit to Old Town Square and Josefov belongs in the same walk, while a visit to the castle adds a river crossing and an ascent. Street and house-number signs may show both a numbered district and a quarter name. Read them as two useful labels for the same location, not as competing directions.