What would be a good use case for the brand new Winter ’21 before delete record-triggered flow? You can see in this example that a record-triggered flow with a loop can be used to replicate a rollup summary field in a lookup relationship. But what happens when a record is deleted in this setup? The answer is nothing. When a record is deleted, your calculation will not update, and your number will be wrong.
With the introduction of before delete record-triggered flow, we can now add a second flow to the setup to trigger an update when a record subject to our calculation is deleted. You can see this illustrated here in this video of a demo delivered to the Seattle Salesforce Saturday audience.
This is also a good example of building two flows on the same object.