We already have an image measuring instrument or CMM. Do we still need a flash measuring device?

Often yes, because the three address different capacity problems. Image measuring instruments and CMMs are accurate and authoritative but slow, so they typically handle first-article inspection, sampling and arbitration of disputes. As volumes rise and full or high-frequency inspection is needed, sampling becomes the bottleneck, and that is where a flash measuring device adds value: single-shot capture with parallel computation brings per-part time below a second, making full inspection capacity-feasible and freeing the measuring instruments and CMMs for the work that truly needs them. A simple test of whether the investment pays off is whether the cost of escapes caused by sampling gaps exceeds the amortized equipment cost.