With this, we now know the exact mRNA contents of the BNT162b2 vaccine, and for most parts we understand why they are there:
The CAP to make sure the RNA looks like regular mRNA
A known successful and optimized 5' untranslated region (UTR)
A codon optimized signal peptide to send the Spike protein to the right place (copied 100% from the original virus)
A codon optimized version of the original spike, with two 'Proline' substitutions to make sure the protein appears in the right form
A known successful and optimized 3' untranslated region
A slightly mysterious poly-A tail with an unexplained 'linker' in there
The codon optimization adds a lot of G and C to the mRNA. Meanwhile, using Ψ (1-methyl-3'-pseudouridylyl) instead of U helps evade our immune system, so the mRNA stays around long enough so we can actually help train the immune system.