The one variable you can count on in a yoga class is how far your feet should be apart. Some say hip width, some together, some the width of the mat, and so on.
I come from a lineage where we do downward dog with legs together. I've had my legs wrenched apart by teachers. Not my favourite thing to happen. They don't tell me why, they just wrench.
And so, here is an idea behind the stance. If you're familiar with another logic please send through, And the idea is for beginners that still work via muscle and bone than prana.
Keeping your legs together keeps the energy in the centre of the body. As a beginner, it is difficult to feel the holistic (all parts together) aspect of an asana. Your mind is usually focused on the instructions by body part, rather than an integrated feeling. Keeping your legs close together is an easy way to integrate until you can energetically feel that idea with legs further apart.
Working with core is really this - not six pack abs.
Taking that idea further via Ayurveda, each body type connects differently to their core.
Vata is the awkward one. She can be physically and mentally lopsided. She requires more centre that say a Kapha person, who would do well to break their centre up. They are hoarders of energy (money, people) so they should actively dissipate. Vata, who likes to wave energy about, should gather in. Pitta, as usual, is the one between.
So instead of a one size fits all yoga class, it would be good to give instructions via concepts than "feet distance apart" and let the punters decide for themselves.
Do you feel all over the place? - legs close together
Do you feel stuck, heavy and self absorbed? - legs further apart