As with this front porch entry, both hardscapes and plantings repetition help create balance and continuity in landscaping. One or more groups or repeated groups will tie areas together better than all single non-matching elements scattered throughout the landscape. See repetition and mass planting for more pictures and ideas for this.
Besides the obvious dominant trees in the picture, there are also a few other examples of repetition in this front yard. And sometimes, like the two Cypress on the front entry, a matching element is almost necessary. Imagine only one Cypress paired with a non-matching plant. The front porch entryway would be less appealing and inviting.