Making preprints publicly available serves many purposes.
Establishes Priority: While many early concerns over public preprints centered on the possibility of researchers being 'scooped' by others taking either their ideas or their data, public preprint repositories timestamp all versions of uploaded projects and, therefore, guarantee a public record of priority for the researcher uploading their work.
Easier Dissemination of Research During the Publication Process: Given the frequently slow pace of peer review and editorial interaction, a public repository enables researchers to make their work freely available to others while it is still in the process of publication. This is especially beneficial in cases where a manuscript requires submission to multiple journals before publication.
Proof of Productivity: Posting preprints also allows for researchers to demonstrate concrete progress to funders and employers.
Why Support Preprints
There are several opinion and blog posts about the benefits of preprint repositories on-line.