Its also highly unlikely that junk DNA would have remained in the genome if it had no purpose
This is a common error; the reverse is actually true. Cells (and by extension, us) serve DNA. Cells (again, by extension, us) are merely vehicles for DNA to replicate - they're the unit of evolution. As a consequence, the DNA does not care whether it contains 'junk' bits. It only cares if it's faithfully copied. Btw, this is an astonishing consequence of evolution that personally blows my mind. If anybody's interested, read The Selfish Gene by Richard Dawkins.
Note: I'm aware that DNA is a mindless macro-molecule. It just easier to anthropomorphism it for the purpose of discussion.