As a guitar student, one way I use my looper pedal to practice improvisation is on chord changes in a progression. This can lead to many different forms of practice, such as scale exercises in a certain key, which can lead to exercising improvisation in that progression and keeping up with chord changes, or if you want to get really jazz-y you can also practice mode changes with non-diatonic chord progressions. Loop pedals are immensely useful for improving my musicianship and I would suggest purchasing one to any student (or at least any student of an instrument that can plug in to a loop pedal).