Is it a must to make BP from A c++?

I have seen some tutorials about class in unreal.Sometimes ,they create A BP class from A c++ .My question is why they do that? And is it A must?

No, most of the time I only do that for widgets. And for debugging, so I don’t have to wait for a long compile.

And also, if you’re working with a team that only knows BP, then it’s better to create a BP that is derived from C++ and use that as the main class.

You can create a fully working game without BP. But as I said, except widgets, animations, materials and particles, even though these are possible to create in C++, it’s advisable to do it with BP.