Even with an extensive metro network, there may be occasions when catching a bus is more convenient and gets you closer to where you want to be. And while it may not be as fast as the metro it provides you with some nice views out of the window.
One of the best advantages of the bus network is the (noctilien) night buses. They start from the time the last regular bus ends (00.30) until the first regular bus starts out (05.30). A cheaper way to get home after a night on the Parisian town than a taxi. Just remember that noctilien buses are marked with letters rather than numbers. Maps can be found at bus stops and shelters.
During the day the montmartrobus (number 64) takes you around the Montmartre district for the same fare as a regular bus. A great way to take in some of the sights.