The jaws of the fly tying vise are a wearing part and they are subjected to the heaviest stress of the fly tying vise parts during tying. With spare jaws, you can easily extend the life of the vise. In addition, many manufacturers offer jaw models suitable for different sized hooks, which makes it easier to tie especially very large or small flies. The correctly sized jaws hold the desired hook size securely during tying.
When attaching the hook to the jaws, you should consider the strength of the tightening. Properly tightened jaws hold the hook securely, but without the use of excessive force. Tightly clenched jaws can lead to jaws quickly bending at the tip or even breaking. The hook should usually be attached slightly behind the tip of the jaws. Larger hooks are recommended to attach further away from the tip.