The DNA that is called junk
is just the DNA that does not code for proteins.
We can identify those segments
by their start and stop codons.
Some of it has a known regulatory function,
but most of it is just old protein encoding DNA
that has been disabled by mutations.
It may really be just junk in the genetic attic.
It is also a raw material for future mutations to start from.
Given that it is not really being used,
it is far less often detrimental when it is altered,
than DNA that is actively in use.
So it accumulates mutations faster
(less failure and death
to discard the mutations).
That way, more elaborate possibilities are possible,
without all the intermediate stages all having to be useful,
when the entire mess gets switched back on,
or transposed to somewhere else, eventually.
Evolution goes faster
when it has junk to play with.