The Function of a Bridge is to serve as a stable guide for the front of the cue . The bridge actually tells the cue where to go. If you want follow, you raise the bridge - which raises your tip. If you want draw, you lower it. Basically, whenever you move your bridge, so will your tip.
Stability is one of the major factors of a proper bridge.
If your bridge moves during your stroke, you may or may not strike the cue ball where you intended. You might even miscue. The bridge should remain the same throughout the range of the stroke. Comfort is another factor. If it's not comfortable then something is wrong.