"NEED" is an confusing word, reguardless of the language....
1.a)We always "solve" Thevenin within the context of Mr. Thevenin. 1.b)Because it no only does a signal "follow" a load, it also follows the supply. + 2) Although we USUALLY just mark down Vcc on the paper, we also are suckers for rechargable batterys. + 3) Because, two people discussing "how well the impedance's preceedances the signal is too much like telling the bartender to "beam me up Scotty." == Because no matter "how you do it", they are all mathematical parts of the current flow and you will get the same algebra anyhow ==<< Because the circuit models the math, and the math DESCRIBES, existing, known things about the circuit, not the other way around.", has been recalled, it is now =+- "because "the algebra teacher forgot to 'isolate the terms' when the current node was Vcc" >>thev "He'll get my tab"./"Marconi not Morse."
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Because we are only allowed to find the ALGEBRAIC awnser, and that involves always solving for a known variable.