The text of your book is printed on a different press than the cover. Whereas a cover press is generally a small format (less than 36"), text presses routinely run from 40" to 55" across. And covers typically require 4 or 5 printing plates, but the text may use well over 20.
While every company is different, usually printers prefer receiving the text as a PDF file (with all fonts and graphics included). Upon receipt, the files are checked for completeness and readability in a preflight procedure. Assuming you've bundled everything together, your title proceeds and the production planning ( a.k.a. book engineering) begins. Paper is allocated, press time is set aside, each department is scheduled, and even the cartons for packing your book are verified as in stock.
Although you probably had to send a laser (lo-res) proof with your files, you are often encouraged to proof the text again from proofs prepared by the printer. If your title already existed as negatives, you could request a dylux (proof), a photo-chemical proof process that actually creates an image the same way your plate would be imaged. Certainly, this inexpensive proof is worthwhile for any changes to your title.
More likely this is your title's first printing, so no negatives will be created. Printing plates will be digitally imaged by lasers. Proofing your text from the printer is more about checking fonts and image placement, especially on non-standard pages like the title, copyright or chapter opening pages. If you aren't experienced in book production, take some time to leaf through your proof. Remember, the next stop is the press.