A few weeks ago I talked about priority inflation in bugs and tasks. It really happens, but it also kind of doesn't matter. You could have P0 - P3, P1 - P4, or P[RED|YELLOW|GREEN|BLUE|BLACK|ORANGE] or any other way of distinguishing priorities. That's because the priorities themselves don't mean anything. They're just a ranking. They have no intrinsic value/meaning.
And once you get past the fact that all priority is relative, and in fact only relative to the other things it was prioritized with, you realize that with a finite set of priorities, your prioritized list of bugs/tasks becomes far less useful. Consider this situation. You've got a deadline tomorrow and you've got time to work on one more thing. You check the bug list and find there are 3 Pri 1 bugs, 10 Pri 2, and about 50 Pri 3. You've got time to fix one of them. What do you do?
You can quickly eliminate the Pri 2 and Pri 3 items because you know the Pri 1 items are more important. But which one? You've certainly got your opinion, and you've probably heard other peoples, but you really don't know. So you pick your favorite one, or the easiest one, or the one that you think is more important. Meanwhile, the one that's really most important (as defined by the product owner/PM/whomever is in charge) languishes. And this can go on for days as new issues are found, added to the list, and prioritized.
So how do you know what's really the most important and make sure that it gets worked on? What about the case where a series of small tasks, each not very important on its own but is more important/provides more value as a group than other single tasks? That's where an ordered list of tasks comes in.
The most important thing, the next thing that should be worked, is on the top of the list. Do that first. If you need roll-up tasks to show the value put them on the list, not the sub-tasks. And keep the list ordered. Frequency of that depends on lots of things. Where you are in the cycle, how fast the team is working through the list, and how fast things are being added to the list at least.
Your list doesn't need to be perfect and as long as the next day or two's items are at the top, the rest doesn't matter all that much. Under the principle of "Don't make any decisions before you need to", make those decisions later. When you have more information to inform the decision. So you can spend less time making and remaking decisions and more time acting on them.
There's another dimension orthogonal to priority, severity. Severity helps inform your decision with information about what happens if you don't address the issue, but that's a topic for a different day.