Why is maximum power transfer based on Thévenin voltage, not the actual source voltage?

[gfd43tg]

Hello,

I was wondering, why is it that when calculating the maximum power transfer, the source voltage is the thévenin voltage, and not the actual source?

What we call the load is arbitrary, no?

 

[NascentOxygen]

Some of the signal is "lost" in that Thévenin resistance, the remainder appears across the external load. Sometimes we wish to maximize the power into the load.