When and why a scientific technical report is written? While making some paper search in my scientific field, sometimes I find articles that are not published in journals or conference proceedings, but as technical reports.

In my experience, I've never written a technical report, and I've never even been asked to do it.

So I was wondering: what's the difference between a technical report and a scientific paper?
Why some researchers publish a work as technical report instead of sending it to a conference or a journal?

When do you suggest to write one instead of addressing it to a conference or a journal?

PS: It seems to me that writing technical reports is more diffused in English-speaking world than in Continental Europe. Is it true? Why?
The main advantages of a research report is that it's published very fast, and without reviewing process. From what I've seen, a research report is basically used: 


to publish a longer version of a paper, for instance including proofs or detailed examples that couldn't fit in a version submitted to a conference. 
to put a timestamp on an idea, in order to be able to claim "We did it back then"
to create a reference that can be cited for project reporting, even though the work has not been published (yet). 
to make a pre-print document freely available, for instance if the published version is behind a paywall. 


Such features are particularly useful when one wants to disseminate (for instance sharing with some colleagues) some unpublished material. Basically, I'd say that most of this can also be achieved by submitting the paper to a public repository, such as arXiv. 

EDIT: considering the language question, intuitively, I would say that when research reports are used in the publication process, it's likely they are written in English. If a non-English speaking university does not offer a mechanism to submit research reports written in English, that might explain why it's not used. However, I've written research reports in France and Italy (in English).