I am setting up UI tests ecosystem integrated with Azure DevOps test plans. I am reading about Specflow v3 and it states that one can use different unit test frameworks with it, xUnit for example: https://specflow.org/documentation/Unit-Test-Providers/
Why would anybody want to do this? To have assertion methods from xUnit? I can't grasp the idea what benefits it would give also I don't get what unit tests frameworks have to do with ui test framework.
You can use SpecFlow to tests everything to want. Yes, it is often used for UI tests with Selenium, but this is not the only use case. SpecFlow is a tool to automate BDD scenarios.
This is our description of SpecFlow:
Use SpecFlow to define, manage and automatically execute human-readable acceptance tests in .NET projects. Writing easily understandable tests is a cornerstone of the BDD paradigm and also helps build up a living documentation of your system.
We support different unit test runner (xUnit, MSTest, NUnit and our own SpecFlow+Runner), because everyone of them has it's advantage and disadvantages. So our users have options what they want to use. Often it is a personal preference what you use. Some prefer xUnit, some NUnit.
Full disclosure: I am one of the maintainers of SpecFlow