I've had journals ask for both. However they claim to only use the LaTeX file as reference in case there are issues in typesetting. In any case, it's still necessary to produce a LaTeX file in order to use the LaTeX template.

Even the ones that will accept a PDF tend to require a PDF generated using their very specific template and style.

Do you mean for review, or even when they've accepted and ask for a camera-ready version? I always assumed all publishers need the source so they can do the small tweaks they do.

[OTOH, I've seen claims many publishers already have a process for publishing Word submissions — something involving monks spending their lives typesetting everything from scratch? — and so when they do get a nice LaTeX, they throw the source out re-use the above process! Perhaps that was only about the bibliography though.]

I've seen some Springer (LNCS) submissions, and those definitely included the .tex source.

arXiv requires the source, mainly so they can re-compile them in the future with better tools [https://arxiv.org/help/faq/whytex]. Unusually, they make sources available for anyone to download.

Earlier this year, I needed to upload the LaTeX file and it needed to be written with the journal's template. In order to specifically fit into the journal's guidelines, the template was sent to some editing contractor where some minor changes where made to the source, e.g. "eq." to "Eq.".

As far as I know, a lot of the journals have their own automated build systems for the manuscripts to perfectly fit into their designs.

Nevertheless, I've also encountered PDF uploads. But even then, the PDF will be automatically checked for consistency.

I believe most (all?) Elsevier journals require the LaTeX source, so you upload the .tex, .bib and any images etc. to their EES wepage, which compiles the PDF there.