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It doesn't need to be checking anything.
Its just telling the server, "Look equip window is open now, look its closed now"
If server didn't know whether the window was open or closed, how would it be able to carry out other functions relating to that window?

Because the window is only GUI that displays the functionality. The server can carry out the functions without knowing when the user had opened window. All it needs to do is be told to unequip the equip, it doesn't care whether or not the user opened that window.