This is an assumption of course, but I think a combination of ease of access (no install, you can just play around, share your results, ask for advice) and seeing the visual flow of energy through the grid. You see how power being generated on one bus is consumed at another. Add a line in between, you see losses. Add a long line, you see the effects of capacitance on voltage and reactive power requirements.

One of my favourite features is the visualization of the admittance matrix and being able to interactively explore how the buses interact with each other.