Why is it possible to replace part of a BJT circuit with thevenin equivalent?

I know it's possible to replace the left side of this circuit with a thevenin equivalent, but on the left we're able to find the base voltage by the voltage divider Vcc * (R2)/(R2 + R1), which is the same as the thevenin voltage.

Doesn't this change the base voltage of the transistor since there's some base current that goes through the Rth?