Here's why:

* Introns are sequences of DNA that are transcribed into RNA but are then removed (spliced out) before the RNA can be translated into protein.

* Exons, on the other hand, are the sequences of DNA that are translated into protein.

So, while introns are transcribed, they don't actually contribute to the final protein product. They are considered non-functional in the sense that they don't code for amino acids.

It's important to note: While introns are typically considered non-functional, recent research has shown that they can play roles in:

* Gene regulation: Introns can affect the rate of transcription or splicing of genes.

* RNA structure: Introns can influence the folding of RNA molecules.

* Evolution: Introns can provide a source of genetic variation.

So, while the term "non-functional" is still widely used, it's not entirely accurate. Introns may have subtle and important roles that are still being uncovered.