Quick research about simulators, please help me understand them

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Hi! I'm a designer working in the Open Source Technology Center at Intel and I'm currently studying about robot simulators in order to improve their usability. I would love to hear from you about your experiences in this area: Which simulators have you already used? What are the ones you currently use and why? What kind of routine do you usually simulate? Why is simulation useful for you in projects? In which moment of the project do you use the simulator? Do you use it during the entire process, or at specific steps?

Thanks in advance, zezefreda