What's the point of this thick trace in my PCB?
The pin size of the QFN is too small to allow such a wide connection without shorting a neighboring pin.  However, the total resistance is reduced by having the trace as wide as possible for as much of the trace as possible.  A typical resistance for a PCB trace would be in the range of 1 mOhm/square.  That means that if you run a current through a 1 cm square of trace, it will be 1 mOhm of resistance.  A 3 cm square is the same resistance irrespective of the fact that it is three times longer because it is also three times wider.  Think of it as three resistors with three times the resistance each, but in parallel so the total resistance is 1/(1/3+1/3+1/3) = 1.

The wide trace takes fewer squares to fill and therefore has less resistance than a narrow trace.