This document, which could be over 130 pages long, is used to secure financing for and lead to pre-production and production of the film.
In the US film production system, producers are typically not allowed to exceed the initial budget.
Director James Cameron ran over budget and offered his fee back to the studio.
While the US system is profitable and can afford to go over budget, some other countries' film industries tend to be financed through government subsidies.
For an example of a budget for a $2 million independent feature, see Planning the Low-Budget Film by Robert Latham Brown.