Some beginning yoga students often come to a studio seeking to look like an image they've seen on social media. Beautiful, lithe bodies with dancer-like extension so many new students think that yoga will help with this outward presentation of their bodies. These kinds of students are often confused when the teacher says something along the lines of, "I don't care how it looks, I care how the posture feels to you." during a class. Other teachers will says things like, "Yoga practitioners can be too flexible, but they can never be too strong." which can also throw new students for a loop. So what gives? Why are so many yoga teachers so focused on strength as opposed to flexibility? Let's break flexibility down from a physical perspective and then talk about how strength is the foundation to gaining more flexibility in your body and not the other way around.
Harvard Medical School says that flexibility allows you to gain full range of motion in your joints which improves your ability to reach for things, bending over to grab something from the floor, or to stoop under things if needed. Flexibility keeps you nimble as you age. Patanjali, the author of The Yoga Sutras talks about Asanas (postures) as a combination of effort and ease. The effort comes from strength, be it strength of concentration, strength in the muscular body, or strength in the opposition between muscles that are relaxed and those that are working in any given pose. The point of flexibility here is clear--flexibility helps with easing into a posture, but strength is needed to keep you in the posture and to protect the body from injury.
From this explanation we can see how focusing on a pretty extension in the toes, or flourish in the hand in a yoga pose actually does very little to build strength--it may look pretty to an observer, but ask yourself is it the hand flourish, pointed toes, or anything that externally "looks" like flexibility building the strength that you need to hold the muscles in place? Does that dancer-like "finished pose" engage the muscles in a way that you can hold the posture for longer than a breath or two? Finally, are you as a student thinking through which muscles are relaxed, which ones that are engaged, and can you use those muscle to deepen the posture? If not, focus on building strength as you practice and pay close attention to what your body is saying as you move.
As we consider Asana and its place in the Eight Limb Path of Yoga and through our study of the Yoga Sutras and other associated yogic texts we discover that the whole point of asana is to prepare the body to sit in a meditative state. Therefore, flexibility is only one component that allows you to sit. Without a strong muscular base, sitting for a long time becomes very difficult. Effort, ease, flexibility, strength are the different sides of the same coin. Your body and your practice will develop and deepen the more you focus on both.