There could be only one purpose for a space vehicle of that size, IF the basic story is true which is a big if; the purpose would be to evacuate one or more cities completely out of our solar system. The ship in the pictures, again assuming the film footage isn't made up, would be one which did not get off for whatever reason.
The flood and the two or three later events which Velikovsky wrote about would have been the final chapter in a series of catastrophes. People living in our system before the series of disasters began would have had no way to know if anything in our system would remain habitable at all; you'd assume at least some of them would try to get out.
THAT might be what an ark is supposed to look like...

For this effect, the main image is a composite of four different frames from the original video taken during Apollo 20 CSM flyover.Thanks.