It’s not uncommon to hear a PM say, “I’d like the development team to estimate how long it will take to implement these features so I can use that to determine how to prioritize them”. Earlier in my career, I would have considered this completely reasonable. After all, if the PM expected a feature to take a few days and the developers estimate that it will take a few months, the feature may not be worth doing.
But now I consider this mentality to be a serious smell that should be investigated. If a PM expects their stories to be estimated between days and months, that means the stories are not broken down into small enough pieces. The stories should be broken down into pieces so small that it would be surprising for any of them to span across an iteration/sprint/etc.
Although splitting stories into manageable pieces is the PM’s responsibility, the PM is not a developer. They usually don’t know the code base intimately. For this reason, it usually makes sense to get a second opinion from a developer if the stories are reasonably sized before the development team points it. Sometimes a developer may know that the prioritization of a few stories needs to be changed for technical reasons, but the PM should have an overall sense of prioritization without getting any feedback from a developer first. Prioritization is the PM’s responsibility, not the developers’ responsibility.
Once you assume that stories are going to be appropriately sized, it’s easy to see why “I want to estimate before I prioritize” is a smell. It means, “I don’t really know the most important thing we should be working on next, so I’ll choose the low hanging fruit”. It either means that stories are not broken down enough or that the PM doesn’t really know what’s important.
As a final note, if it’s safe to assume that the stories are going to be appropriately sized, the level of effort of a few days should not make the return on investment questionable. The features that would provide mere days worth of return on investment should never be considered as a priority in the first place.