Daimyo were powerful warlords that owned their own province in Japan. They had complete control over military and economy in their province. The provinces were similar to gangs in a city (see below). There were about 260 daimyos in Japan. Daimyo even got to have their own castles where they lived and where they did their business. The daimyo had a large impact on the feudalism of Japan.
Daimyo came in power in the tenth century and stayed in power until the early nineteenth century when Japans feudalism ended. They first came in power because they raised secret armies of samurai and eventually came too powerful that Japan lost control of them. Without this increase in power from samurai, daimyo, and shoguns, the feudal system of Japan wouldn't exist.
The daimyo would take over a piece of land and give protection to samurai and peasants, as long as they were loyal to the daimyo. This is a big reason of why they were able to become so powerful. It was like a symbiotic relationship since the daimyo gave them land and/or protection, and the person getting it were loyal to the daimyo by serving them. This relationship was called a lord-vassal relation.
Although it wasn't entirely daimyo that caused the feudalism of Japan, they still played a big role in it. I believe that daimyo affected Japan in a big way.