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It helps if you know what myrrh was used for at the time.
Myrrh and frankinsense along with "costly spices" were given to the Christchild as gifts. Myrrh as well as frankinsense are natural remedies, both of them being resins from trees native to the Middle East and northeast Africa.
Myrrh can be used to treat a number of conditions including heart, arthritic and other ailments. It is also a painkiller. Myrrh and frankinsense are also embalming agents. They won't preserve the body long term but will help mask the odors of death. The Gospel of Mark records that the sour wine Christ was offered on the cross was a mixture of vinegar and myrrh.
Christ tasted that wine and rejected it. He took the punishment in full, without relief. That's one aspect of the symbolism of myrrh. The other, the gift at his birth, foretells both of his death, and that in death he would be buried as a King.
There is even more to it, but those of the main takeaways for why myrrh is mentioned in the Bible.