Reason for minimize PCB tracd width?
Impedance control (discontinuities), and parasitic capacitance (two sides of the same coin really) if you are running over a ground plane.
In addition, the larger a trace is, the higher the chance you might have to narrow it down to squeeze it past something and that introduces an impedance discontinuity. Sometimes it's difficult to tell in advance if you might run out of room and if your trace is too big you have to reduce it down which might mean you have to re-arrange a bunch of stuff.
Also, it can make it more difficult to solder. More heatsinking. I guess this could also cause tombstoning in some cases if you have a small component with landings sitting right on the a giant trace.
Also, have you ever routed a board? It can be mind numbingly tedious to keep on changing trace widths. I used to try and make my traces match the width of the pads, but after running into so many traces that run to multiple components each  with different pad sizes I threw my hands up in the air and just used the same 0.3mm trace width for every signal when it was not critical.