why soccer players stretch

​Passive stretching of calves significantly improves ankle range of motion, balance, jumping, control and agility.

Like most professional athletes, the Matildas use dynamic stretches, (swinging their limbs around, bending, lunging etc) to warm up, as well as longer hold, passive stretches after training and games.
Passive stretching is when you stay in one position and relax your body while a partner, prop or even another limb intensifies the stretch by putting external pressure on your body.
Recent research on 'The Effects on Flexibility in Amateur Female Soccer Players' 2023 involving a comparison of those who only did dynamic stretches, those who also did passive stretches and a control group (no stretches) showed those who did long hold stretching with 3 sessions per week for 4 weeks significantly improved their ankle ROM (range of motion), drop jump performance and running agility.
So make sure you regularly do long hold stretches with assistance (whether that's from a stretch therapist or a prop of some kind) to improve your flexibility.
In terms of timing, make sure you stretch ideally after some exercise or most importantly, not before any strenuous exercise as it can impede performance and increase risk of injury.  
These are two great calf stretches that target both the lower and upper calf:
Standing Calf Stretch – Gastrocnemius: Stand arm’s length from a wall and place one foot forward, keeping both feet flat on the ground. Lean forward towards the wall until you feel a stretch in the calf of your back leg. Then to activate the fascia, push your foot of the straight leg into the ground and towards the wall several times.

​Kneeling Calf Stretch – Soleus:  Kneel down and place the knee that's on the ground beside your foot, keeping your other foot flat and heel down. Then lean over your knee without lifting your heel. Then to activate the fascia push your flat foot down into the ground and behind you.