Well, if you don't have anything to say to the Journal Office, then the logical approach is to just leave the field empty. And if you there was something you wanted to say, then that's the place to put it!
A typical thing people might want to put there is that they think that a particular editor would be a good fit for the article you're submitting. Another example would be if you think that an editor is negatively pre-disposed to your work or has any other kind of conflict of interest, or if there are reviewers for which this is true and that you don't want to be involved in the paper.
But if you can't think of anything that you want to say, then don't say anything :-)