Vets will have a security clearance. This is the biggest drawback for a straight civilian. Your security clearance stays with you for life.
If you apply for a military contractor job, say Afghanistan, the first question they will ask is if you have a current security clearance.
The other reason is that vets know the system and can be up and running with minimal training.
Not a vet, but I got a clearance about 30 years ago. Realistically it is a week of work at most for whoever is doing the background investigation. Ought to take another week to review and approve (add a few dollars for private database access). The value of clearances is inflated mainly because the system is vastly less efficient than that. Takes many months and dollars to get cleared nowadays.
Yes and no, once you get you clearance you have to be reinvestigated every 5 years for TS and Q and every 10 for S and L. Also, atleast in the defense business, your clearance can be downgraded. I've seen folks go from TS to S simply because the contractor did not have work requiring a TS.