Visual biofeedback is important for two reasons:

1) the pelvic floor muscle is difficult to control voluntarily (on purpose) - visual biofeedback allows the user to confirm or disconfirm that the correct muscle is contracted.

This information is vital in order to quickly and correctly learn to voluntarily contract the pelvic floor muscle.
2) Biofeedback shows the user how much the pelvic floor is contracting. This allows the user to measure the strength of the pelvic floor muscle as well as progress.

The ability to objectivly measure progress has been shown to motivate users. Accurate biofeedback provides certainty that the exercise is useful and successful.