I think you have to look at the evolution of the home music reproduction system to understand the Pre-amp's role. It is the result of the need for a single point of control and access. It was about control as in signal path switching, balance and 'tone' alterations, level setting, It also provided a cost effective and systemically benign location for an RIAA equalizer and some additional gain for it. In those days the production of music sources and the recording technology combined with early component left a lot to be desired. The need to 'balance' the early stereo effect was present along with the need for the classic Base and Treble personalization of the listening experience - It all resided in the Pre-amp.
Times have changed a bit.
Times have changed a bit.