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Statistical Significance - simulation methodologies relays on data volume only on the training part, where the 'customer behavior' has been analyzed and different segments have been created.
Lower Cost - while real-world testing relies on users volume(marketing is the biggest spent for online companies), in the sandbox approach, the Cost moves to IT(ML) and relies mostly on data processing, training, and execution.
No-Risk - applying a test to a live and running game contains risk - underperforming test results, time-consuming, in PvP games, the latest can break the fairness mechanism.
Nonstop Innovation - I see that a lot, that once considering making changes in monetization features or economy objects in general, there are many concerns of 'braking something' or even disrupting a working money generating process.