Do you want to know how your page looks like with your future pictures? I find it prettier when my photos match with each other, so that my page looks clear. Of course the photos matter, but I think the overall look of your profile is just as important.
When someone decides to go to your profile, has seen your profile picture, your bio and then sees your last 6 pictures, it should be clear, it should match and look pretty. It´s not necessary, but I always love clear profiles.
So I always use this app before I start with posting a new serie (I work with series, like Jordan, Portugal, Spain etc.).
You can add a lot of photos and swap photos in the app, just to check how your future posts will look with each other.