Why does the subscription key have product=<name> when I select a Product?

I noticed in the APIM Test tab, it has <subscriptionKey>product=<productName> for the Ocp-Apim-Subscription-Key when you select a Product from the drop down.

Why doesn't it just set the subscription key to be whatever key is set for that Product?

The test console uses a master key that does not change if you select a product, so instead of using the product subscription key, it just uses its name, so it does not impact the quota of the product subscription (if you use the quota or rate limit policy) while testing.