It's not for research purposes. It may prove useful for that in the long run, but the recommendation includes eras and places that you have no interest in writing in. Indeed there's enough primary source floating about that it would be a trick and half to sample a lot of it and manage to use it all as research. That's not its primary benefit.
Its primary benefit is to knock your block off.
The past is another country; they do things differently there. By reading widely in all sorts of pasts, you start to get a grasp of not just what this society is like, or that one, but what societies are like. That things you take for granted are not something you can rely on in other societies. That, in fact, things you take utterly for granted are weird and abnormal as far as cultures go.
Primary source is this because while it doesn't let you inside their heads, it gets you a lot closer than secondary source does, because secondary source also has all the reading between the lines elements that primary source does, with the added issue that your secondary source's issues are not information about the primary source. If the list of crimes that a person is charged with is completely over-the-top, it does indicate that hurling mud at your opponent in hopes some would stick was practiced.
To be sure, secondary source is primary source about the era it was written in. Reading about the assemblies of the ancient Germans in Victorian works -- especially if the writer is pushing the Teutonic origins theory of parliaments, to give democracy a venerable history -- or the rights of the stout Anglo-Saxon ancestors that were lost in the Norman Conquest may tell you rather less about the ancient Germans but quite a bit about the Victorians.
And good secondary sources -- the technical kind, aimed for historians or at least knowledgeable amateurs -- can cite primary sources you might not want to wade through yourself. True, they're selective, but you'll have to be selective, too; there is not time enough to read all. A writer who abstracts from early Christian sermons the advice they gave about how to become like a little child, and presents it as evidence of what positive traits little children were deemed to have, saves the time of reading all the sermons.
But while that helps, it's no substitute for reading primary source.
It may even be useful for having done your research in advance later. It certainly helps alert you to the possibility that you should do some.
Though the point at which I thought I might have gotten a pretty good feeling for how a society fits together was when I got back a critique that indignately asked what sort of world chaperoned a princess and let a prince ride free -- and my first thought, popping instantly to mind, was "A normal one."