I just picked up a work by Herder and he uses italics for practically every other word. I have noticed this in other sources I have read, usually from around the 18th-19th centuries and I can't understand why certain words are stressed. I realise that it is probably for emphasis but I can't tell why he is emphasising words so frequently. Take this sentence: "One gets closer and closer to the fortunate climate where one human couple began spinning the thread -- under the mildest influences of the creating Providence, with the aid of a most facilitating fate all around ..." etc. He is obviously writing for an intelligent audience yet his use of italics makes it read like a children's book. I think if I knew why this convention existed it might make it easier to get through what is already a very confusing work.
First of all it should be mentioned that Herder published his original works in the German states in German, using Fraktur, as was the tradition, and Fraktur doesn't have a concept of italics. But in the original editions of his works he did accentuate a lot of words and terms with bold, which I guess was translated into italics in your English translation.
It is important to stress that this was not a common thing for 18th-19th century writers, this was strictly Herder being Herder.
Even in his own time he was reknowned for his particular way of writing, not only in his choice of words but in his stylistic approach as well. His way of establishing his philosophical vocabulary was by repeating what he considered key words and phrases again and again, not just in his various works, but within the same work in varying examples, and he further highlighted these key words and phrases in bold so as to make sure the reader understood that they were key concepts.