The more items we have in progress at once (WIP), the less important our initial prioritization is. When we work on one item at a time then items get done in the order we started them so we are completing them in the order of most importance.
When we have 15 items to do and they’re all in progress at the same time then it doesn’t matter when we started them, the smaller ones will finish first. Meaning that it didn’t matter if we prioritized them up front or not. They get completed in descending size order, not order of importance.
Why does this matter? Many companies spend a lot of time doing prioritization up front and then lose all the value they would have got from that because they don’t control their work in progress. All that prioritization becomes waste.
If you’re still skeptical and want to see the proof for this, see this excellent Drunk Agile podcast.