1. persistence, which obviously is hurt by them running out of money. Dropbox links apparently die eventually, static websites bitrot, etc. Things put on arxiv can be trusted enough to put links to them in papers, if needed.

2. Discoverability. Pre-print archives send out digests (usually people subscribe to particular areas). It's a low-effort way to distribute them. Related, by having things semi-centralized, it's easier to discover papers (they get indexed by google scholar, for example...).

Maybe other benefits? Not sure.

https://duckduckgo.com/?q=preprint+server+site%3Anature.com&... for example lists many previous publications by Nature on the topic, including https://www.nature.com/articles/nmeth.3831 https://www.nature.com/articles/s41467-017-00950-5 and https://www.nature.com/articles/d41586-019-00199-6 . (The first was not in the flagship journal "Nature" but a sister journal.)

In short, a preprint is a version of a scientific paper before it's been formally accepted by a journal. See https://en.wikipedia.org/wiki/Preprint .

The first real electronic preprint repository is https://en.wikipedia.org/wiki/ArXiv . That gives more background information on the topic.

abought describes some of the differences between a set-of-files and a preprint repository at https://news.ycombinator.com/item?id=22319686 .