A canary release is used when you create a new version but you want to only give it a small percentage of requests to test it for bugs before migrating 100% of requests to it.

For example, give a new version 10% ot the requests and split the traffic randomly. You can then monitor for errors. If there are no errors, you can gradually increase the percentage. Eventually, you can give the new version 100% of the requests.

If it turns out your new version has flaws, you can easily migrate 100% of the traffic back to the older version.