They are not garbage sequences iirc, they control gene expression. DNA is a string that is coiled tight several times over itself (can't explain better than that, see the video below :)). So the garbage sequence is like padding that pushes genes in and out of areas that can be read. Changing amount or composition(?) of the garbage can turn a gene on or off just because it becomes inaccessible to RNA replication mechanism.

https://www.youtube.com/watch?v=bW5JnYZImJA