They are trying to get a good spot for “Hanami”, a cherry blossom viewing party. If you don’t secure a spot in a famous “Hanami” place beforehand, you won’t be able to have a cherry blossom viewing party.
This is a kadomatsu (門松, かどまつ, gate pine). It is a traditional Japanese decoration for the New Year. Japanese people set two pairs of Kadomatsu in front of the gate of their house to invite “Toshigami” (年神, としがみ) inside. “Toshigami” is one of the Gods of the Shinto religion in Japan.