>what is the linguistic process or reason for joining words when there is no difference in meaning between the old and the 'new' form?<

Lucy, I don't think this is going to help at all, but some languages don't separate words with spaces when writing.

Someone (Bingley? I forget) made a post a week or two ago about 'what is a word?' I think this may be relevant here.

If the writer thinks of a word as 'a unit of speech expressing a unit of idea' (which I am not necessarily saying is the corect definition, by the way!) then on occasions when a 'unit of idea' actually requires two words in English, that writer might be tempted to join the words so as to get down to one 'unit of speech'.

When I look at the examples you quoted, they all seem to make sense from this point of view.
Personally, I detest them, but I'm trying to understand the writer's thinking.