As a visualization aid.
So if there are ancient aliens somewhere, maybe this is how they're doing it.
Von Neuman machines are even less likely considering they would still have to remain fully functional over a truly geologic timescale in order to be in any way functional. If you had technology that robust, it wouldn't need to be self-replicating because it would basically last forever. It's a Crazy Eddie concept that otherwise serves no practical purpose.
It's mundanely obvious that interstellar travel is perfectly feasible for an unmanned probe that can expect to be discovered intact after a billion years or so by an alien race that didn't exist when it was launched. Travel or transport LIVING BEINGS just isn't going to happen, though, without a space fold or something similar. Or -- as Gary mentioned -- if a civilization evolved near the galactic core or in a dense cluster of stars, in which case interstellar travel only takes slightly longer than interplanetary travel.
Still. greater numbers means two things: You still have more machines out there after falling prey to attrition through disaster, wear and tear, etc. Also more machines means covering the distance at sub-light more quickly, spreading like a virus.
First of all, they wouldn't spread more quickly
at all. They would spread more EVENLY, but still take millions of years to do it.
Second of all, even a virus requires a viable host to reproduce itself, as to Von Neuman machines require a source of plentiful and accessible resources to use for self-replication. Interstellar space has no such resources, and even if it did, they are hardly in a form that self-replicating machines would find easily accessible.
Lastly, the concept of the Von Nueman machine is one that is only theoretically viable on a relatively small scale -- say, mining a moon or an asteroid or something. Using them to spread your influence across an entire galaxy is a bit like trying to build a suspension bridge out of legos and superglue: an amusing hobby, but hardly practical.
Yes you can expect organic beings to make it across interstellar distances, but more realistically by the methods i named. Its perfectly feasible to recreate humans once we meet up with our robotic explorers.
You can say that if and when we have discovered a way to keep frozen embryos viable over a span of half a million years.
It would take as little as 500,000 years with Von Neumann probes. A long time for us but minute in cosmic history...no time at all really.
DNA: In fact, DNA is a hardy information storing medium, lasting millions of years as well as being hardy when stored. It is now being experimented with as a storage medium for archival use. Sending DNA we can spread humanity through the galaxy, letting us start new civilizations. If we can combine it with "brain uploading/downloading" if such technology is available, then we can even reproduce the same human being.
Interstellar space is not "empty":
Outside the Solar System
Looking beyond the Solar System, there are billions of potential stars with possible colonization targets.
The long-term survival of the human race is at risk as long as it is confined to a single planet. Sooner or later, disasters such as an asteroid collision or nuclear war could wipe us all out. But once we spread out into space and establish independent colonies, our future should be safe. There isn't anywhere like the Earth in the solar system, so we would have to go to another star.
— Stephen Hawking, Physicist