Take a distinctive combination of words: "her labile yet exigent demeanor"; "as non-descript as a sparrow in the suburbs." When you are paraphrasing a secondary source, your own vocabulary will overlap somewhat with that source, but you wouldn't want to use a phrase like that from the source you are citing without quotation marks. An especially distinctive or felicitous phrase has more of a claim not to be repeated without attribution. By the same token, you would not say that, 'according to Fulano, HItler "invaded Poland" in 1939.' The quotation marks make no sense there, since neither the idea not the words have any claim to be distinctive. Putting those words into quotes would imply that you were commenting, in some way, on that particular combination of words.
Human language is creative in the rather ordinary sense that we can invent phrases and sentences that have (probably) never been used before. For example, I could find no examples on google where labile and exigent were even in the same vicinity in a sentence. Even a string of ordinary words can be distinctive: "As the cat stepped over the top of the jamcloset, first the right forefoot, carefully, then the hind, stepped down, into the pit of the empty flowerpot." Sure, there is a torrent of words on the internet, but if you express your own thoughts precisely, trying to say exactly what you mean, you are unlikely to duplicate the previous efforts of any other writer.
There are also facts that are banal and repeated from source to source with no change. Somebody was born in such and such a place in such and such a year. There is a standard wording for that, even.
I hate when people cite my banal statements. They have every right to do it, but I'd rather they cite me where I'm making a good point rather than for background.