During the Lich King encounter, the platform that you fight on crumbles at the edges, reducing the area in which players have to maneuver on all sides. From then on, Val'kyr mobs (3 in 25-man, 1 in 10-man) will periodically spawn, pick up members of your raid, and fly towards the edges of the platform. If the Val'kyr mobs are not killed before they reach the edge, your raid members will be dropped by the Val'kyr to their deaths. In order to buy yourself as much time as possible to kill the Val'kyr, it makes the most sense to position your raid at the center of the platform. Unfortunately, while this is occurring, the Lich King will periodically target a member of your raid and cast Defile. Defile causes a black pool to form under the feet of the target and deal shadow damage to anyone who comes in contact with it. To make it worse, every time any player touches that pool it will increase in size, and the damage dealt will increase as well. So, if your raid is stacked at the center of the platform when a Defile is cast, everyone has to scramble out quickly to avoid enlarging the pool, while continuing to DPS the Val'kyr. On top of all this, the phase is based on the Lich King's health, so you have to whittle his health down as well, or else the phase will never end.
So where does the bug come in? It's pretty simple: if players can rebuild the Frozen Throne to its original state, you will have more space to avoid the Defile. This might not seem like a big deal if you've watched the 10-man kill video from Blood Legion, but add in 15 more people and it quickly becomes a much more difficult aspect of the fight.
Another advantage to rebuilding the platform comes from the Valkyr. Ensidia has stated openly that by having the platform extension present during the phase, it was unnecessary to DPS the Val'kyr at all. See, the Val'kyr do nothing in the fight but carry players to the edge and drop them, so if the Val'kyr were just dropping raid members off over the rebuilt edge of the platform (as opposed to over a deadly drop) then this mechanic of the fight is completely trivialized. It would simply disable three of your players for a short time. This allows you to rush through the phase, focusing all your DPS on the Lich King.