Many regard this plant as falling into the ‘cottage garden’ category only, but it is far more versatile than that.
I would use these to kick start a plot in the springtime as they are pretty plants anyway, although the flowers don’t last very long, the leaves are delicate and hide bare patches of ground whilst allowing light through.
They look really good against fluffy foliage, Fennel, or Achillea for example, or they can soften harder edged plants like Sisyrinchium or Iris.
I don’t think they look good next to heavy leaves though so I avoid planting them near to Hostas, Hellebores or Bergenia for example.
There are loads of different varieties, but they tend to cross breed with the more common variety, so mine have all tended to end up with purple or pinky purple flowers, but that’s ok by me as I like purple.