Academia is itself self promotion. Conferences, publications, talks, all of it.

I cannot speak for the intended purpose of ArXiv by its creators but I can tell you that, in the conference circles, its main intended use was flag planting: people were afraid that their competitors would tweet some results while their (earlier) paper was under anonymous review, and so researchers started putting their stuff on ArXiv first to ensure no one would "steal" their claim of being there first.