Many otherwise professional publications and websites suffer from not being proof-read properly. When silly and avoidable errors slip through it makes the business involved appear sloppy and unprofessional. A thorough proof-read before publication is the final touch that shows the business pays attention to detail in all its activities.
Proof-reading is about much more than spotting spelling mistakes: it entails identifying factual errors, ensuring that pictures are correctly captioned, that page numbers agree with the flat plan, that pictures are the right way up (yes, it can happen!), and cutting over-matter without damaging the sense of the copy. It involves marking up clearly to avoid misunderstanding with designers or editors, and checking revised proofs to ensure that amendments have been made.