[mcfunk]

Yes exactly -- it's two different sources of information that one party is not likely to have access to. Severity from a technical-only perspective is important to have. The sense of whether something is easy to fix (which should be size) is also important.

Ultimately you could have a low-severity bug, but it impacts a really important customer who cares about it, so it's high priority. This is the most common case I've seen.

Severity also can impact whether something gets prioritized at all. If it's low-severity and the business side isn't there either, maybe it just doesn't get done. Whereas something with high severity but low business impact still probably needs to be done regardless or it will lead to bigger problems.