Flattening makes filled-in form fields, comments, stamps and drawings part of the page, so they can no longer be edited or removed. It also makes sure the PDF looks the same in every viewer.
Interactive parts such as form fields and annotations are merged into the page content, so they become ordinary, non-editable content.
Yes with Forms and comments. With Whole pages to images, each page becomes a picture and text can no longer be selected.
Before sending a filled-in form, a signed document or a reviewed file, so nobody can change the answers or comments.
No. Everything happens in your own browser, on your own device. Your files are never sent to a server, so nobody else can see or keep them.