People can read the abstract and then decide if they want to go deeper and also download and read the PDF. I’m sure that many only read the abstract.

Furthermore, depending on publishing site, a paper may also be available as HTML rendered from the LaTeX source, in addition to PDF. (If the page does not now, it may in the future.)

The purpose of a [PDF] tag is to warn about possible unsuitability of the linked resource for mobile consumption (which isn’t the case for the article page linked here), possible download size (though maybe not anymore, nowadays), and possible brightness shock when using dark mode.