Ethical reasons: because we should not thoughtlessly displace (and perhaps render extinct) the species that evolved here.

Ecological reasons: because other species may depend on them.
Karner Blue butterflies are endangered because wild blue lupines, their host plants, are too few and too scattered.

Practical reasons: because native species are well-adapted and will survive with less care than non-natives.

Esthetic and spiritual reasons: a garden that fits its habitat gives the gardener or visitor a sense of harmony with nature.

Economic reasons: a functioning ecosystem filters the air and rain, restores oxygen to the atmosphere, moderates temperatures and wind speeds, holds and slowly releases water instead of allowing it to race downhill and erode the soil. Suburban lawns and traditional gardens, by contrast, may protect the soil, but do not absorb much rainfall, heat, or wind. They require chemicals, which pollute the ground and groundwater, and power equipment, which pollutes the air. When we try to replace natural systems with artificial ones, we do it at great expense and far less effectively.