Why does Ivanka need a residence in Washington?
Your guess is as good as mine, and the apparent non-necessity of her moving there has people concerned that she will stick her nose in where it doesn't belong at her dad's invitation, which would definitely be a problem.
Some freepers have speculated that her husband will have a spot in the administration; but others have said federal nepotism laws also rule out in-laws; so I don't know. Someone said she should oversee the Trump Hotel in DC, but given the large numbers of properties they manage around the world, that really isn't necessary, either.