Some plants look right together because the gardener planned it that way. In my case happy accidents are often the cause of plant combinations that work well. In yet other gardens it is the growing conditions that are the over riding factor that create the overall effect.
The Flag Iris above are in the process of being colonised by the small Euphorbia which spreads rapidly by creeping roots. In spring I am happy that the red shoots contrast with the lime-green, sword shaped leaves of the Iris so I plan to leave them until the Iris flower. By then the dark blue flowers will not need a red leaved under planting so they may have to come out. These Iris also need to be split as the rhizomes are old and each piece only produces one stem. I will do this splitting a few weeks after flowering and plant pieces of root and throw away the old woody center.
Acid soil loving plants often work well together partly because the soil conditions are a bit more exacting. I plant winter heather amongst the Rhododendrons so that colour is continuous from autumn through spring. I also have Mahonia near-by and Anemone Blanda in the loose peaty soil..