Imagine the scenario where, given 3 days to work on it, a developer can get the deployment time for their code down from 3 hours to 20 minutes. This company does, on average, 42 deployments per year (you can guess these numbers are totally imaginary).
So 3 days at 7.5 hours per day means we are investing 22.5 hours on this.
The return is the difference between the deployments, multiplied by the number of deployments that are needed. So 3 hours is 180 minutes - so we save (180 - 20) = 160 minutes with each deploy. We do that 42 times in a year so we've saved 6720 minutes (per year) which is 112 hours or 14.9 days.
Project managers might not like to lose 3 days from their schedule but how do they feel about having a spare 3 weeks each year?