The mRNA 5ʹ cap regulates splicing, nuclear output, resistance to exonuclease degradation, and initiation of translation. In short, 5' cap has the following functions:

Regulation of nuclear export;

Prevention of degradation by exonucleases;

Promotion of translation;

Promotion of 5' proximal intron excision.

https://mrna.creative-biolabs.com/mrna-5-capping.htm