Toilet tank lids should be flat. Perfectly flat, not merely sort of, kind of flat.
The tanks in our house appear to have flat lids. This is deceptive, a trick of the bright white color and lighting of the bathrooms. Closer inspection reveals a slight convex curvature, a bump in the middle of the lids.
You place a book, a cellphone, almost anything you decide not to hold and do not wish to set on the floor, on the lid of the water tank. The item slowly, imperceptibly, slides from the bump. It finally crashes to the floor, precisely the location you sought to avoid.
We don't yet have shelves in the powder rooms and they don't have sink counters; the first floor has a pedestal sink. There is no good place for items, because the one surface that should be flat (and safe) is not.
After we get around to painting and finishing the baths, I'm adding some shelves. We can place candles and decor on the shelves, but I'm making sure there's room for a book or two, as well.