Bank statements and utility bills routinely arrive password-protected, which becomes tiresome when you need to read them regularly or combine them into one file for an application.
If you can open the document, this can save a copy without the password. Restrictions on printing and copying — the so-called owner password — can usually be removed without knowing any password at all, because that restriction is advisory in the PDF specification rather than cryptographic.
That distinction is the useful thing to understand. A user password actually encrypts the file's contents, so it genuinely can't be read without it. An owner password leaves the contents unencrypted and merely asks readers to disable printing or copying, which is why removing it is a matter of rewriting a flag rather than breaking anything.
The practical case for this is mundane: statements that arrive protected with a password derived from your date of birth, which you then have to type every time you open them, and which can't be merged into a single file for an application until the protection is gone.
Type into real form fields, or anywhere on a flat form.
Draw, type, or upload a signature and place it on any page.
Number the pages, with control over position and format.
Add DRAFT, CONFIDENTIAL or a logo across the pages.
Cut away white margins or crop pages down visually.
Combine files into one PDF and set the order before you merge.
Break a document into separate files by range or interval.
Drag pages into a new order, rotate or drop the ones you don't need.
Turn sideways or upside-down pages the right way up.
Pick pages to drop, visually or by range.
Keep only the pages you want as a separate file.
Combine JPG or PNG images into one PDF, with page size control.
Render pages to images at the resolution you choose.
Shrink a PDF with a side-by-side quality preview and a size target.
Add a searchable text layer to a scanned document.
Extract text as plain text or Markdown, in reading order.