It's important to remember that stretching your lower legs involves more than just the main large calf muscle (gastrocnemius), in the back of the lower leg just under the knee. The soleus muscle, located in the lower half of your lower leg, is between the large gastrocnemius and your achilles tendon at the ankle. The only way to reach the soleus muscle is by bending your knee while you stretch your calves. When you stretch them with straight knees, you'll be primarily stretching the gastrocnemius. Keeping the soleus muscle as well as your achilles tendon strong and flexible is important if you want to keep running or even walking! Think of your lower leg and ankle as your first line of shock absorbers when you run or walk. Keeping this area healthy takes a huge load off your knees, hips, and back.
In this stretch, the standing leg has a softly bent knee, thus stretching that soleus. The extended leg is getting a little stretch action in the hamstrings as well as gastrocnemius and achilles tendon. Just pull up on your toes and push your heel forward on that extended leg while pushing down into the ground with the standing leg. The more you can keep your lower back from rounding, the more you can get a hamstring stretch out of this, too!