My idea is , when we want to substitute an exercise instead we are searching for the exercise we want to substitute , it could already be the option with the exercises that we can do.
For example if there’s a hip trust and we don’t want to do hip trust , not everyone knows the alternative for each exercise , but if you click on substitute and it already appears only the exercises that can be an alternative to hip trust instead we have to scroll to every single exercise.
I hope it maked sense :)