Low-impedance tap in this oscillator circuit
It is the nature of oscillators that the oscillation will tend to grow in strength until it is limited by something.  In this case that something is the voltage supply working against a fully turned-on transistor in series with the emitter resistor.  To get the maximum power transfer, the impedance seen by the transistor collector needs to be in the neighborhood of 27 ohms (give or take a factor of two).

Thus the low-impedance tap.  Because the crystal is operating into the transistor's stray capacitance, it's impedance as seen by the LC tank is going to be quite high.