Carp streamers are typical decorations seen leading up to Boys' Day (more recently called Children's Day). When the wind blows it looks like the carp are swimming upriver, which is a sign of a strong fish. Families should get a carp streamer for each boy (now, child) to hope for children to grow up strong. Many families buy new streamers every year. This is partly because new things are seen as better in Japan and partly because nobody has space to store things with such small living spaces.
Of course, like many Japanese cultural points, the celebration is rooted in Chinese culture, but the commercial aspect is purely Japanese.