A vast majority of problems call center tech support deal with are a direct result of the user not understanding what the computer is actually telling them ( not understanding/not reading error dialogs, assuming a software problem is a hardware issue or vice versa, in general not knowing how to use a computer, etc)
If you can't adequately explain what your problem is in a way a machine can understand, then there is no hope that a machine can fix your problem for you. If you *can* explain your problem in a way that a computer can understand, then you can almost always fix your problem yourself and you don't even need a help desk.
If there's an issue that you can explain adequately but don't have the ability to fix yourself (resetting passwords in a corporate environment, etc). Then there might be a use case there, but how is that meaningfully different from enabling self-service for those tasks?
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