A "new vulnerability" is not necessarily a new security risk; it just means that they were recently discovered or disclosed. The risk could impact old and existing code and has potentially been present for a much longer period of time. This means you could have new known vulnerabilities even without making any code changes!
By allowing Snyk to monitor your GitHub/BitBucket/GitLab repositories, your deployed code in PaaS / Serverless environments or your local projects means you will be the first to know if you are affected by a newly disclosed vulnerability. You can then assess and act upon the vulnerability risk quickly.