"pdf" stands for "portable document format" and it was created so you could read a document without having installed the program that created it.
You'll really love that feature if you download a document created in MS Word to your phone that doesn't have MS Office installed, so you can't read it.
And it certainly makes a lot of sense for publications like e-books and brochures that are made with professional programs you can't even pay. There's no need to change an e-book or the brochure of your favorite holiday destination, is there?
And then there's the well-known fact that details in the lay-out of an MS Word document depend on the printer installed and of course on the presence of fonts used in the document. There's no single guarantee that your Word document will look the same on some other PC. While a pdf looks and prints exactly the same everywhere. If that ain't a good reason to use it!
To what end was it created for?
I don't understand how some people insist to use PDF files in emails, which is meaningless, since the minute it is sent off, the file stays UNCHANGEABLE unless the server breaks/gets hacked.