Academia isn't my thing but I also wonder if there isn't an aspect of putting a stake in the ground? So that if someone beats you to publishing you at least have some record of being on that track.

That is definitely a big motivator for publishing preprints. Journal submissions can take up to a year. Comference submissions take months. If the field is moving fast, claiming a finding early can become an important career move.

In older days, academics would just share notes on their work and word wouldn't usually spread widely before publication.

Preprints may be the better model. But public visibility means that non-experts now get to see the good and the bad research equally, but they won't have the domain knowledge and skill to distinguish one from the other with confidence.