Why has the auteurist theory become so important to film analysis?
Auteurist theory has become important in film analysis based upon the fact that many have denounced filmatic adaptations of other original pieces based upon their decision not to follow the original text. Essentially, auteurist theory allots the director the freedom to direct a piece based upon their own personal vision of the film.
Today, films (both adaptations and originals) need to be critiqued and analyzed in their own rights. Each film needs to be deemed original (regardless if it is an adaptation of an earlier work given the new version has been created independently (only influenced by the original if an original exists).
What this means, when one boils it down, is that everyone is entitled to their interpretation of life (texts and art included). If a director fails to include specific ideas in an adaptation, that director should not be ostracized for it. Instead, the director should be scrutinized based upon his or her own ideology and depiction of the film itself. Nothing else should be taken into account but the film itself. Essentially, the film should be regarded as if it, itself, is an original.