There are three muscles groups that every runner should stretch on a regular basis. The calf muscles are one of them.*
The calves include the deeper-lying thicker Soleus, and the more superficial double-headed Gastrocnemius. Both of these muscles merge into the Achilles tendon.
The Achilles, as all tendons in the body, is not elastic, meaning it is a set length, incapable of being stretched beyond its length. To stretch a tendon is to tear it. Hence, the only way to give some relief to the Achilles tendon from any tightness is to stretch its attaching muscles - the Calves.
Why it's important to runners: After a run, the calf muscles respond to the exercise by getting tighter. If not relaxed (being massaged, rolled, and stretched), they will remain tight, creating a strong continuous pull on the Achilles tendon. After the next run, the same thing happens. But now, an even stronger tug is being applied to the tendon. And so on, after each run. If not relaxed or stretched at some point, the Achilles tendon is not able to maintain its length and begins to tear and become inflamed (tendonitis).
If you are not a runner, the calf muscles typically get tight as well. It is generally a good idea to stretch and massage them.
The most effective way to stretch the calf muscles is to step one leg back, plant the heel fully on the floor, and hold the stretch for at least 2 minutes. The long hold will actually create more length in the calves. Holding for less time will not be as effective, but is still beneficial to relax these muscles.
Straight-leg stretch primarily targets the Gastrocnemius muscles, while a bent-knee stretch (heel still planted) targets mainly the Soleus.
Please note, if you have a tear, or tendonitis, stretching deeply will only make it worse. It's best to perform Eccentric Heel Drops first before performing any long-hold stretches. (Rule of Thumb: Stretch only healthy tissue).