How a project plan is like a musical score
An orchestra conductor uses a musical score to direct a piece of music. Every instrument has a different part to play. It's in the way that these parts work together that music is created.
Each instrument has its own part. The sheets of music that each musician uses tell exactly when, and what, to play. The musician knows how to play already - after years of studying, practice, and performance.
The conductor uses the musical score to know all the parts. He knows exactly when, and what, each instrument needs to play. The conductor has spent many years, usually, studying music. His emphasis has not been on the in-depth study of an instrument, but on the interpretation of the music. And on how to lead the musicians who make up the orchestra.
In the same way, a project manager "conducts" the project. Using a project plan, each member of the project team knows exactly when, and what, to do. She is able to perform the required work because of her knowledge, skills, and education.
The project plan is made up of many different parts - all the specific plans like communication, risk, quality, etc. - so that the overall project accomplishes it goal(s). The schedule and budget are integral parts of the project plan, but not the only parts.
Each member of the project team is interested only in the part that they contribute to the project. The project manager is the one who's interested in making sure the project dove-tails together at the end.
By using a project plan, the project manager is directing the project. Utilizing the knowledge and skills of the project team members, the project manager is able to successfully complete projects.