Urithiru is by far the best place to base and coordinate against the Voidbringers. It's been implied that it's always the base of operations during a desolation.
Once the rest of the Oathgates are restored it is literally the most connected place in all of Roshar. They will be able to travel all over the planet to battle the Voidbringers. It's quite possible that it will actually be faster to get to Kholinar / Alethkar from Urithiru than it would have been to march from the Shattered Plains. Especially with two sets of storms to deal with.
The city also likely holds a huge amount of information about both the Knights Radiant and fighting voidbringers.
It's also mentioned in chapter 6 that legend claimed the city to be protected from Voidbringers.
They moved at the end of the book because they knew that the Desolation had come, the Everstorm was coming back (towards the 'leeward' side that's less protected), multiple highprinces were dead and the army needed to recover from their climactic battle. If all that wasn't enough, the Alethi no longer had a reason to be on the Shattered Plains - the Parshendi had been defeated with survivors scattered. The Vengence Pact had been fulfilled and it was time for the kingdom to move on.
Dalinar had also been trying to unite the highprinces and end the near complete separation of warcamps. Uritithiru gives him a chance to do that.