The Composter will help your Farmer get crops faster. The Composter will turn items into compost using the compost barrel. The Farmer will use the compost as fertilizer (like vanilla bonemeal).
Compost is also used to create compost blocks, which are used in the Flower Shop schematics.
The Composter can also make dirt in their compost barrels as well, which is especially useful if you are in a biome without dirt. When producing dirt, the Composter has a small chance to produce podzol instead. Researches at the University can increase this chance.
The higher a Composter’s Stamina level, the faster they will compost. Similarly, the higher their Athletics level, the less time it takes for them to fill a compost barrel.