Užupis as a district is famous in Vilnius first of all for being a 'free republic' with its own mayor inside the city. The district even has its own constitution. And this bridge leads to this spectacular 'country'.
Moreover, Užupis is said to be the best place for artists to gather around (there's even a famous book written here about bohemian artists in love). So this bridge in particular is famous as it keeps a mermaid (creature, half fish, half girl) beneath it. Truly there is a mermaid! She is sitting under the bridge just across the river.