What is the function of registering DLLs- What does registering a DLL do?

problem ：

I know how to register dlls but I've never really been sure why I'm doing it or under what conditions a dll must be registered. I know how to register dll But I'm never really sure why I'm doing this , Or under what conditions must we register dll.Could somebody explain or point me to some documentation? Can someone explain or point out some documents to me ？

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Solution ：

Reference resources ： https://stackoom.com/en/question/2bVq