Why is it important that scientists use cDNA in the plasmid instead of "regular" DNA? What it the difference?

cDNA stands for "complementary DNA".  It's DNA that's been reverse transcribed from mature mRNA.

Okay, eukaryote DNA (we're eukaryotes, by the way) has segments in it that don't really code for anything useful.  These segments are called introns.  The segments between introns that DO contain useful code are called exons.  Normally, when DNA is transcribed to make mRNA, the mRNA then goes through processing to splice out the introns and glue the exons together.  The result is a piece of mRNA that is ready for translation in the ribosomes.

Prokaryotes (bacteria) don't have introns, and they don't have any mechanisms for removing introns from eukaryotic DNA.  Hence, if you want to insert a human gene into a bacterium, you have to remove the introns yourself.  But how do you do that?  You take a segment of mRNA that has already been processed, and reverse transcribe it into cDNA.  The cDNA lacks the introns that were part of the original DNA and is ready for insertion into a plasmid.

I hope that helps.  Good luck!

This is very helpful. Thank you