It's the 3' (3-prime) UTR (un-translated region). It can affect the translation of the mRNA.

https://en.wikipedia.org/wiki/Three_prime_untranslated_regio....

The next thing is the poly-A tail:

https://en.wikipedia.org/wiki/Polyadenylation

blasting the 3' UTR, we see it ~50% of it was copied from the human mitochondria

tldr, extra regulatory signals (often not well understood)