My understanding of this is that if someone is an obvious criminal, it will be easy to prove in court, so the attorney will not change the outcome.
For example, if the guy who ran over and killed the woman in Charlottesville didn't get fair representation or trial, both sides could rightfully argue "the system is rigged."
[1] https://en.wikipedia.org/wiki/John_Adams#Counsel_for_the_Bri...
It's also the key insight of rule utilitarianism over act utilitarianism.
Because even the worst criminal deserves his due process rights, a fair trial, etc, etc; this is fundamental to our American society. It is a cornerstone of our judicial system.
Unfortunately, like much of American society, it's a theoretical pipe dream that falls down flat in reality as minorities don't get access to due process, fair trials, etc.
Without defense attorneys, nobody could be "proven" guilty because a show trial proves nothing.
If you want someone to be eligible for punishment, you should thus WANT them to be defended.