The numbers on keys can mean a couple different things and it is important your are familiar with these. In some cases the numbers are a way to identify which key blank you have. But in a lot of cases the 5-6 digit number can be the DNA to your key.
Why is it important that you know what this code means?
Many people (especially young people) will post pictures to social media of the keys to their new/first apartment or house. They are caught up in the excitement and likely not thinking about the security vulnerabilities with doing so. If someone sees the picture of their key and the code that is stamped on it, a new key can be generated as an exact replica.
In addition to this, lets say you are at the library reading a book and your keys are on the table. Somebody could walk by and see your key’s DNA code stamped right on the key.
To protect yourself against this vulnerability, I recommend getting a duplicate key from your local locksmith or hardware store. A duplicate key will not have your keys bitting stamped on it. Then you can put your code cut key with the stamp in a safe place at home.