What is the role of a fungus's fruiting body?

The answer is a production of spores which are used for reproduction.

In fungi, the fruiting body contains spores, which are the fungi's reproduction units. Spores are released by wind, rain, or animals. Once they land on an adequate surface they grow into hyphae, and eventually form a mycelium. The mycelium can later develop into the fruiting body and, thus, the cycle is repeated.