Concurrent versus consecutive prison sentences - rationale

Rarely do I see convicted criminals serve consecutive sentences when they’ve been found guilty of two or more crimes. Almost always, they serve their sentences concurrently (In fact, I don’t think I’ve ever seen anyone handed consecutive sentences*). Why is this?

Are there guidelines or criteria that are used to help determine which outcome should be selected by the judge? One factor that does seem operative is whether the crimes themselves were committed, essentially, cuncurrently i.e. part of the same act or set of acts. Am I correct? Are there others?

*I suppose it’s possible that this is a strictly Canadian phenomenon and, if so, I apologize for generalizing - you can then ignore me.

I don’t know how they choose, but the reason for the difference is the same for multiple sentences existing at all- in case one gets overturned. Suppose you were convicted for 6 months and 12 months. 3 months into it, the 12 month gets overturned. If serving concurrently, you’ve only got 3 months to go. If not, then you still have 6.

And the main reason for concurrent at all, rather than always consecutive, is that often it’s really the same event being punished. Like waiving a gun in public and breaking parole by holding a gun.

http://www.justice.govt.nz/pubs/reports/1997/sentence_guide/chapter_8.html

In the US at least I had always heard that consecutive vs concurrent was tied to parole considerations as well. If you got two ten year sentences running concurrently then you could get parole after serving some portion of the concurrent time. For a consecutive sentence even if you got parole for (or served out) the first sentence then you still had to serve out the second, and in some states having a consecutive sentence meant you weren’t eligible for parole on the first one at all. Parole and probation have undergone a number of revisions at the state level over the last several years so I don’t know how accurate if at all this is.

On a tangent, how is it determined which sentence is served in what order? In the situation posited upthread, the same person had two sentences and one conviction was overturned. If the convction that was overturned was for the first sentence, does the prisoner get any time served credited to the second sentence?