The Walls of Ba Sing Se protects almost your entire board from destruction. Its static ability gives your other permanents indestructible, covering creatures, artifacts, enchantments, planeswalkers, and lands. The Wall itself is the exception, so protecting it becomes an important part of protecting everything behind it.
Artifact decks can deploy it through effects that put expensive artifacts directly onto the battlefield. Goblin Welder can exchange another artifact for the Wall from the graveyard, while conventional ramp can support casting the eight-mana creature. Because the protective effect is static, it begins working as soon as the Wall is present; there is no activation to pay for first.
The broad protection pairs well with destruction-based sweepers. Your other permanents can survive effects that destroy the relevant types, though the Wall may be removed if the sweeper affects it. Indestructible does not stop exile, sacrifice, or a creature dying from zero toughness. Nor does it prevent planeswalkers from losing loyalty, so the effect should not be treated as universal protection against every kind of removal.
Defender limits the Wall's normal attacking role, making it primarily a support permanent rather than a finisher. A deck using it should already have threats or engines worth preserving. Additional protection for the Wall, plus a clear way to win with the surviving board, makes the eight-mana investment more purposeful than simply adding a large defensive piece and hoping the game slows down.
