I find them useful for cueing and beatgridding, etc. I can't really put my finger on exactly why I like them, I just do.
I imagine only from rekordbox, but don't take my word for it. As I understand it, a scrolling waveform is too much info to stream accurately in real time without latency via USB. That's why controllers don't often have screens. If manufacturers started building hardware for USB3 or thunderbolt, however, a LOT more is possible.
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