Put yourself here.
You are out for a drive in the car. You stop for a break, smoke a tab. Get on your way.
A week later a nutter dumps a body in the hedge.
Body gets found, SOC move in - you get a dawn raid.
Well members of the Jury. Hair was found at the scene, the DNA led us to Mr bollocked via the database.
When we looked in his garage there was the means of transporting the body - the car even has mud matching that of the scene.
Where were you on the evening of - err at home reading a book, out for a drive. Can you prove that, no - oh dear thats life for you then.
Oh did you hear about bollocked, yeah never would have thought but he was bang to rights the scum bag.....
At present you have to be a suspect for real reasons and DNA is there to assist final ID - this gives some protection. With a database you just become the first to get nailed, crime gets solved figures look good.