Why does Poe go into such detail when describing the external features of the House of Usher? What symbolic implications do the details have, particularly the fissure that runs through the edifice?

The details the narrator provides about the crumbling and decaying house foreshadow the fate of the crumbling and decaying Usher family; the "House" of Usher, then, refers to both the physical house itself as well as the Usher line. The narrator remarks specifically on the "barely perceptible fissure, which, extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn." In other words, a casual observer of the house would not necessarily detect the weakness in the house's structure.
Likewise, a casual observer of the family would not necessarily detect the figurative cracks or weaknesses in it, but a more detailed look reveals what will lead to its demise. Roderick Usher tells the narrator of a "constitutional and family evil," a strange malady for which there is no cure. "He suffered much from a morbid acuteness of the senses": in other words, the way he experiences textures, odors, and sights causes him real pain and "horror." Roderick even entertains a superstition that his fate is somehow connected to the house's fate, and he sullenly anticipates the death of his sister from illness as well. It becomes clear that the family disorder also has a component that affects one's mind as well as one's body. Poe provides some hints that the reason this illness has developed and run rampant in the Usher family is because the family has been incestuous. Roderick says that when his sister dies, it will "'leave him . . . the last of the ancient race of the Ushers." He can, if he chooses, have children with other women, so his certainty that the line will die when his sister does seems to be based on the idea that he was expected to have children with his sister. Perhaps it is this family expectation (and his horror of it) that causes him to bury his sister alive. He then dies when she turns out to be alive.
A casual observer of the family, then, will have no knowledge of the family's practices, especially if the family disease causes the family to remain within the home. Just as the home has declined, so too has the family declined. When Madeline reappears, having escaped her grave, she and Roderick both collapse, as does the house. They are all too fractured to remain whole.

As the narrator approaches the House of Usher on horseback, he describes certain features of the house as though they possess sentience. The windows are "eye-like" and "vacant," and the house is covered in fungi that seems to hold it together in its crumbling condition. A "pestilent and mystic vapor" pervades the atmosphere of the house's exterior, and the overall effect of its appearance is foreboding. Poe sets an appropriate scene for a horror story about how incest has brought a once prominent family to an ignominious end.
Just like the family line that has always lived in the house, the house has come to the end of its structural integrity. Roderick and Madeline are the end of the house of Usher line, and the family estate has a "barely perceptible fissure, which, extending from the roof of the building . . . down the wall in a zigzag direction" foreshadows its imminent collapse.