I've used both Insomnia and Postman and simply put, Insomnia is to Postman what VSCode is to Visual Studio.
Postman's power comes in its ability to do more advanced prototyping and API development stuff and "project" or collections of API calls that are exportable and sharable.
Insomnia, on the other hand, is more strictly for interfacing with existing APIs, and as a result doesn't really have feature parity with Postman, but that's not really a bad thing. Not everyone needs or wants the advanced features of Postman and just needs something to poke at an API with. For that, Insomnia works great.
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