Instead of "I have only ten hours to get all my stuff from place A to place B", it's more like "I temporarily stored 90% of my stuff in easily accessible place C in my spare time, so now I have ten hours to get the remaining 10% of my stuff over to the new place before permanently saying goodbye to the old place".
Again using my "moving apartments" analogy. If you can store the stuff you don't immediately need for everyday life somewhere else, it reduces the critical time of permanently switching your primary base of residence over to the new place because there's less stuff to move when the time comes.
Well, my main point is that it will speed up the system migration so I have A) less system downtime because there are fewer files to transfer before the new system can be up and running and B) it reduces the risk of something going wrong during the system migration.
It's like you find out that there's a fire going to break out in the house you live in soon, so you find a new place, get your entire Transformers collection into safe storage, and now all that's left are the empty cabinets which you intend to move over to your new place soon. If the fire breaks out before then, sure, you'd need to get new cabinets, but your toys are already out of the danger zone.