Why use replaceNull when setting a value in Nifi updateAttribute

[#1]

New to Nifi here. I was looking at this post. that talks about how to make a retry flow. In it there is a updateAttribute that sets the variable `retry.counter` to ${retrycounter:replaceNull('0'):plus(1)}. How come they set it to that instead of 1? This is the beginning of the flow and should always be 1, no? A similar thing is done with retry.maxcoutner - `${retry_maxcount:replaceNull('3')}`

Please help me understand the logic behind this.

Thanks!

[#2]

@Micah Pearce

There are numerous ways to accomplish the same result in NiFi.
The use of the ${retry.counter:replaceNull('0'):plus(1)} NiFi Expression language statement is to create an attribute with value 1 if attribute "retry.counter" does not exist on the FlowFile; otherwise, the existing value currently assigned to existing attribute "retry.counter" is returned and incremented by 1.
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If we just set it to one, each time the same FlowFile passes through this loop it would get set to 1 and would never increment.

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Thank you,

Matt