Importance of knowing GC Content of an organism
Interesting question. The GC-content seems to evolve over time and it also seems that the GC-content of coding regions is higher than for the surrounding non-coding regions (see reference 1). If there is a specific function for this higher GC-content or not is (if I understand this right) debated among the groups which do research in this field. Have a look at the references (and probably also their references) to decide on this:


Both selective and neutral processes drive GC content evolution in
the human genome.
Integrating genomics, bioinformatics, and classical genetics to
study the effects of recombination on genome evolution.
Recombination Drives the Evolution of GC-Content in the Human Genome
GC-Content Evolution in Mammalian Genomes: The Biased Gene
Conversion Hypothesis
Contrasting GC-content dynamics across 33 mammalian genomes:
Relationship with life-history traits and chromosome sizes