In order to allow for more autonomy allowing clients to alter exercises in phase. Not just substitute in that particular workout so the change remains for the duration of that phase.

This way a client can swap out exercises when needed from a program, rather than having to substitute every workout or you can supply a program that calls
Quad Dominant - Hi Bar Squat ( can swap for Hack Squat, Front Sq)
Horizontal Row - BB Row (Can swap Tbar Row, Seal Row)
The client can change the exercise which allows you to offer the program to a wider audience with alternatives. Takes away the need need to deal large numbers of individual changes which can be very time consuming.

This also allow for equipment availability to be dealt with easily as Personal Trainers tend to deal with a wide variety of people with specific equipment options.