A little bit of a historical note. Erwin Chargaff, a biochemist, performed many detailed chemical analyses of DNA from various sources and always found two rules to be true. 1) The number of moles of pyrimidies were always equal to the number of moles of purines. 2) The number of moles of A was always equal to the number of moles of T (%A = %T) and the number of moles of C was always equal to the number of moles of G (%C = %G). Of course, we now know this comes about from the double helix structure of DNA and the base-pairing rules of DNA. But, Chargaff's observations came before any of this was known, and his experiments were crucial to the discovery of the double helix structure of DNA. In particular, they gave Watson and Crick the crucial insight that T pairs with A and G pairs with C. This insight, along with the x-ray diffraction work by Wilkins and Franklin which said that DNA adopts a helical structure, gave Watson and Crick all the information they needed to propose their famous model for the structure of DNA.
So the fact that %A = % T and %C = %G is important because this is led to the discovery of the base-pairing rules.