I have had jobs from my demos alone!
One that comes to mind is through Sony Music... doing spots running in India for various albums/groups. That was a pleasant surprise.
Also, the first Save the Date wedding trailer I did was based on a demo. That gig led to a press release from Stephanie, which lead to at least three other Save the Date type jobs.
And, I frequently hear that a client liked my demo but contacted me for other jobs they had in the future. The demos do sell my voice!
