Proofing traditionally got performed manually by reading manufactured hardcopy proofs before the files reached the printing press.
Nowadays, we have much more content across many more platforms than traditional print. Web content, video, audio, and interactive files are commonplace in the modern world, and none of those formats can get checked using physical proofs. Many companies have multiple output formats and need to cater to all these different formats in the proofing process.
The file could get printed, published on a website, social media, or broadcast on TV. Proofing has evolved, and Online Proofing is the most cost-effective and environmentally friendly method for review and approval.