Standing side splits is one of many favorite exercise in the reformer repertoire.
This exercise strengthen leg and hip muscles, improve balance and improve standing posture.
Often times the teacher cue to have weight even across the foot, or towards the little toes in order to lift the medial arch of the foot.
Now it is time to go and examine your shoes ... do they ware out on the little toe side, or the big toe side ?
It is fairly common to notice shoes waring out on the lateral (little toe) side, in that case cueing to have more weight on the big toe might be beneficiary to correct the pattern of distributing more weight on the lateral side of the foot while walking.
In addition, transferring weight to the big toe will help activating the Abductors of the hip: Gluteus medium, Gluteus
and Tensor Fascia late.
Distributing weight even across, or towards the little toe, would help activate the opposite muscle group, the Adductors.
The beauty of the Pilates method is in it's options, depending on your specific needs the teacher might cue one way or the other.