Flattening pdf files is useful when somebody sends you a pdf form to fill in.
Yes. If you've ever received an a form in an ODT file, you'll see the text after the characters you type move to the right or move down the page when reach the end or hit 'enter'.
If information is entered into a table, the characters you type and the text in the form won't move until you reach character limit in that field. Depending on the formatting, the boundaries the cells will do one of three things (at least those that I know of in my experience editing and creating PDF's.)
Either the characters you type run past the boundaries and you don't see it, nothing in the table is shifted.... But I need to tell you that stuff.
None of those things happen in a PDF that is flattened. Filling out a PDF is literally like inserting a page into a type writer. If a layer is locked, you have to click or tab over to that area before you can type in it. If you keep hitting space, and I'm not totally positive which it is, either the cursor disappears behind the text box ( a layer) or 'returns'.
If you've ever converted a PDF into word format, You'll see all the tables used to create the document. Editing a PDF that's been converted into an ODT file can be confusing. Take any PDF to the Adobe website. They convert it for free. Open it in Libre and you'll probably see what I'm talking about.
I start PDF in Libre Office then export the ODT file as a PDF then flatten it with something else. It might be possible to flatten it in Libre Office Draw, but Draw skews things in PDF's so I haven't bothered.
After you’ve filled the form in, if you flatten it before sending it back, it means that nobody can easily edit the file to change any of your personal details that were on the form. Because in the flattened .pdf, the fields in the filled-in form are no longer editable. They’ve been flattened down to an image……. I think!
I could be wrong, but it’s something like that.