There are two known gene products, RNA and protein.
The regulation of gene expression is important because in different moments of human deveopment different substances are needed and also during the stable fase of human development (adulthood) necessities are dieverse.
The regulation of gene expression is what makes cells and an tissues different and that is why it's also so important.
This question is indeed not east. Hope I suplied enough information according to your needs (there is still way more info on the subject).
Hope it helped,
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