With my Necrons, being a Warrior heavy foot based list I will often use a refused flank deployment model- especially if the table is a bit light on terrain.
Refused flank means you pile up all your models on either the far right or left side of the table- protecting one side of your army, and often your more valuable mission model units by the edge of the table.
With my Necrons my Warriors blob out and my Destroyers are on the table edge- especially against these Ultras as the Vindicators are the biggest threat to the Destroyers and they need to be protected until I can deal with the Vindi.
Refuse flank also means that being in a corner your opponent has to come out to the center of the table to get you- unless they have enough long range units and volume of dice to just pound away at you all game- which this Space Marine list doesn’t have.
RF pulls the marines in, and my Warriors want to be in the center of the table so it get’s them there quicker.
Difference between a refused flank and castle deployment is that with a refused flank you still plan to move out and take other parts of the table- castle up means you intend to stay in place for the entire game.
For the Star Gods!