What do we mean by plants the perform double duty?
Plants that do more than look pretty are essential to a thriving, balanced garden. It is the backbone behind any thriving ecosystem both domestic and wild. This is part of the 90% planning and 10% doing idea.
The best example of this that I can think of is food. Growing your own food is the basis of Eat MY Shrubs, and the inspiration for the name. Incorporating food producing or medicinal plants into your garden space is a great addition. Additionally, plants that are good for pollinators, or that attract beneficial predatory insects, are great too. Plants that are nitrogen fixers for the soil or provide shade, are a few others.
Both native bees and honeybees are fans of long-blooming sunflowers. You can harvest the seeds or leave them for songbirds. They will serenade you all winter as they enjoy the energy filled snack.
However, if there is an issue in your space, there is probably a plant out there that can help. Some evergreen, some not, thorny or no, produce fruit or repel insects. The more functions that your plants perform in the garden, the less we have to.