You always get mutations because nature isn't perfect, and organisms are complicated. The genetic code accounts for that so that if you get a single point mutation, the probability that it still codes for the same or a similar amino acid is quite high. That means the protein it codes for can probably still function, which is all nature cares about.

(This is very, very simplified)

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There isn't really a significance. There are 20 "ancestral" amino-acids, while the genetic code uses triplets from 4 distinct nucleo-bases (ACGT) which yields 64 codons. Redundancy is unavoidable.

Now, why did the genetic code evolve based on triplets... that's kind of a million dollar question

However, there are significant or interesting properties and consequences of redundancy. Maybe this was the question you had in mind?

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