Even thought you use the grids in your view finder of your camera (depending on the model you use of course) it will happen that sometimes you image will be tilted. Just because you didn’t pay enough attention to it.
This is why the straighten tool in Lightroom is one of my favorites.
It can also happen that you did pay enough attention to the grids and it just looks a little bit off afterwards. Just that tiny bit. Then this tool comes in very handy.
Usually I look for a sign post, a lantern or something like that in an image where the tilting could be an issue.