I don't generally support the death penalty because it's too easy to make mistakes that can't be undone.

To me, the value of death penalty with reprieve is that it incentivizes people who might receive it to make full confessions of other crimes. The phrasing is that you get a life sentence if you are not found to have had committed other crimes within 2 years. The past tense here is important.

Say I get convicted of taking bribes and it looks like I might get a death sentence with reprieve. If I've also, say... Forged some documents in the past then there's an incentive for me to confess to forgery so that it can't be used to put me to death if someone finds out about it in the next two years. I'm already going to jail for the rest of my life, but if I confess to all my crimes at least I won't be executed