Why In Figure E7-1 the two resistors run from the supply rail to ground with their junction tied to the transistor's base. That divider sets a fixed DC voltage at the base, which (minus the roughly 0.7 V base-emitter drop) sets the emitter voltage and therefore the quiescent collector current. This is the classic voltage divider bias network used to place a transistor stage at a stable operating point that does not drift much with temperature or beta.