Its already been said, but when you use a mechanical release your arrow flexes up and down. This is not a good thing when shooting off of the shelf or any other rest that puts tension on the arrow from the side (springy, plunger, flipper, etc.) This is why the majority of compound shooters use a rest that supports the arrow only from the bottom....and recently the trend has gone even further, so that the rest moves completely out of the way upon release.
When you release with your fingers, the arrow flexes side to side, which is how you are able to tune setups that aren't centershot so that there is no contact with the plate at all upon release.
Were you to try to tune your setup with a release, you could get it to shoot bullet holes through paper, but your arrow flight would be horrendous when you switched back to fingers.
Scott