You could set limits per number, have it lock to just single merchant, etc. pretty nifty when paying some wacky merchant online.
All have since shuttered the service because pretty much every CC comes with purchase protection that you can invoke to charge the vendor back in case of something going wrong.
Virtual CCs provide very limited utility in my mind - because the place you're likely to have your CC swiped - a bar or a cab - are still going to use only the legacy plastic version.