"Wir vergleichen ja gern und oft."
Just like your other recent question, this modal particle isn't easy to explain. I would suggest that "ja" supports the factor of trueness, like: We compare, we really do, happily and often. At the same time, it says "as you already know" – maybe because it is widely known or common sense even: "Sie wissen ja, wie Kinder sind" – or "as I mentioned or suggested before". And that just might be my personal perspective and someone else might find other tones in this single word. There's no way I could translate it.