Boring Sci-fi people. The cooler explanation is that the "junk DNA" is a design for some future race of mankind.
THE REAL REASON FOR JUNK DNA:
Each gene produces one protein. So, if cellular transcription occurs at a fairly steady, environmentally controlled rate, how do organisms make more or less of a given protein?
They make COPIES of the DNA. DNA doesn't actually get used to produce protein, directly. It serves as the template for mRNA. The more protein needed, the more mRNA is produced.
Redundant copies of DNA also have the same benefit, but gene B is a copy of gene A, either gene A or gene B can be destroyed, without the likelihood of the organism being impaired. Also, viruses insert their genes into DNA as a way of replicating themselves. Typically they pick up portions of human DNA when they do this. This is actually quite common, and it is how gene therapy typically is done.
(This info was what was believed as of my college education, which was over a decade ago.)
IMHO, the "junk" DNA actualy does quite a bit. Scientists simply haven't figured out what it all does... yet. Heck, "they" don't fully understand the brain or exactly what triggers the process of natural labor or what triggers type 1 diabetes... I could go on.