Proofreading is important. Many times the document you are producing is the first, perhaps your only contact with the reader. Their only frame of reference is what they see in front of them. When they've read your piece, they will decide if you've made your point. Revision and checking for errors will make the piece understandable. Subconsciously or not, they've also made a few judgments about your competence.
If the piece has many errors, the reader may decide that you didn't care enough to proofread. They could also suspect that if your writing isn't well done, you don't know what you're talking about.
Formatting is used to apply a standard look and feel to documents and provides the reader with an effortless experience. Good formatting feels 'invisible' and the reader can focus on content.
If the proofing and formatting is not targeted for its audience, misundersandings could result. For example, documents with information on building a school. A school board will want documents in economic terms an in a certain document format. Financial institutions may need some of the same information in another form like spreadsheets. An official giving a presentation on the subject may use a slideshow.
As a writer, I feel that doing my very best to provide the easiest reading experience shows my concern for the subject and respect for the reader.