The producer can come up with the idea or option/acquire the material; the foundation. The producer then hires the key "employees" to help execute his/her vision. The writer (if the material is based on but not limited to; an idea or adaptation), director, DP, talent, etc.
After the producer let his/her "employees" do their job and get the film in the can, the producer still has a lot of work to do. The producer has to "sell" the product/service (the picture) to customers (the audience) through marketing. Here's an obvious statement; marketing can make or break a film. Remember, the producer is not just selling a film, he/she is "selling" a company. Is it a bust or a multi-billion dollar company? Your next CEO position depends on it.