Actually, it's more about efficiency and called the "wobble position". Here's a wikipedia paste since i'm lazy:
In the genetic code there are 4^3 = 64 possible codons (tri-nucleotide sequences). For translation each of these codons requires a tRNA molecule with a complementary anticodon. If each tRNA molecule paired with its complementary mRNA codon using canonical Watson-Crick base pairing, then 64 types (species) of tRNA molecule would be required. Since most organisms have fewer than 45 species of tRNA, some tRNA species must pair with more than one codon. In 1966 Francis Crick proposed the Wobble hypothesis to account for this.