Was the Bible written by men who were inspired by God or was it written by men who were telling tall tales, motivational stories, or trying to deceive in order to gain something for themselves? Were the authors of the books of the Bible who they claim to be? Because if they were not then we have a problem, how did men who were either delusional or deceptive write a book that, apart from authorship, contains evidences of divine origins? That is, how did writers who were lying or deceived get the details correct? That is one overall evidence that the Bible writers were authentic.
It is not possible to actually confirm that any particular individual wrote a book of the Bible, apart from what the Bible says and what has been passed down from tradition. There are no signatures to compare, no fingerprints, just copies of what they wrote that have been passed down through the centuries. What we have is archeological evidence and old manuscripts.
So if we can't be sure who wrote the Bible, then how did the critics of the 18th and 19th centuries figure out that five different writers wrote the Pentateuch, or Daniel was written by a 2nd century BC imposter, or Isaiah was actually two or more people? Without getting technical, they did it by looking at the text and deciding whether it showed consistent enough writing styles to be written by a single individual. They also looked for anachronisms and other inconsistencies that might indicate that the traditional writer could not have been the actual writers. In the end however all they have is their analysis. Archeological and manuscript evidence still favors the traditional writers.
What is the purpose behind attributing the authorship of the Bible to a number of unknown writers? The main purpose is to try to make the Bible look fraudulent. The second is to try to explain why the Bible contains fulfilled prophecy. If you can demonstrate that the Bible was written by men who were not what they claimed to be, and if you can demonstrate that prophecy after an event happened, you can put the Bible aside as un-authoritative, and of solely human origin. But in reality, that is not what we find.