I sort of understand the reasoning on why Arxiv prefers tex to pdf[1], even though I feel it's a bit much to make it mandatory to submit the original tex file if they detect a submitted pdf was produced from one. But I've never understood what the added value is in hosting the source publicly.
Though I have to admit, when I was still in academia, whenever I saw a beautiful figure or formatting in a preprint, I'd often try to take some inspiration from the source for my own work, occasionally learning a new neat trick or package.
A huge value in having authors upload the original source, is it divorces the content from the presentation (mostly). That the original sources were available was sufficient for a large majority of the corpus to be automatically rendered into HTML for easier reading on many devices: https://info.arxiv.org/about/accessible_HTML.html. I don't think it would have been as simple if they had to convert PDFs.