One immediate issue is that they are probably not important to most people in the world, but of great importance to McCracken herself, something that many of us would feel about letters from members of our own families.
Elizabeth McCracken, a fiction writer born in 1966, has written several admired works of fiction and been awarded important literary prizes. One of her books is a memoir detailing her pregnancy and the stillbirth of her child. Her family letters and connections become especially important to her in the absence of the child she was expecting, filling part of the void she experiences.
Literary fashions, however, shift, and the popular or admired novelist of today may not be of any interest a few years from now. When people collect and publish the letters of historical figures, it is done under the assumption that if we are, for example still interested in Cicero two thousand years after his death, it is probable that people will continue to read his letters. Whether McCracken's work and correspondence will endure over the long term is not something we can judge at the moment.