Very often, I find commas placed after introductory phrases that actually strongly affect the meaning of the main clause and would not be marked with commas in a complex sentence. For example, commas are very frequently being used in: "In Enlgand, the weather is often bad", "In the summer, we get our GCSE results" or "Every year, we go on a family trip". Sometimes, I even find apparently 'introductory' elements that the main clause's verb is dependent on, such as: "From this, we can see that..." or "In my mock, I got...".
What I have come up with is this:
A phrase that none of the main clause's parts of speech are dependent on but that the overall main clause is should be considered a separate dependent clause.
However, this still doesn't explain the latter bit, and, having attempted a research on my hypothesis, I found nothing.
Can anyone explain?