So basically, we have a schematic-level understanding of cellular processes, but to see the actual 3D interactions in realtime would be extremely illuminating.
Cells are like little cities in terms of the complexity of their biochemistry. We want to ask questions like "How does this cell respond to this chemical/drug/change in environment."
Imagine trying to understand in detail the gun crime epidemic in a city, if you can only see objects larger than 100 m on a side. You wouldn't see people, cars, or many buildings.
We want to be able to understand, explain, predict, and control cellular process, but so far we have to be quite indirect. Understanding these things at a mechanistic level, in realtime would revolutionize our ability to understand, repair, and build biological systems.
maybe it'll one day help with cancer research.