[bartlettD]

Piggybacking onto this comment, but another reason for external pours is thermal performance. A copper pour on the surface on the PCB allows heat to convect off the board more easily. The gains aren't massive, but they can help as part of a larger thermal management scheme.

I've also heard, possibly apocryphally, that in the old days when we used harsher chemical etchants, removing all of the copper from unused sections of the PCB would increase the risk of thinning the traces beyond what was intended. So in those cases a copper pour would reduce the time the PCB would need to spend in the etchant bath.

[petsfed]

I've worked with several stepper motor driver ICs that feature a ground pad on the bottom of the IC and recommend an unmasked copper pour connected to the ground pad via thermal vias, on the opposite side of the board from the IC, sized at least as big as the IC itself. Like, the manufacturer's suggestion is literally to use the copper as a heat sink. If you wanted, you could then affix dedicated heat transfer features, like a traditional finned heatsink, or a heat pipe to a dedicated cooler.

[exmadscientist]

Thermal pads are wonderful. Most of the time for moderate thermal loads, an exposed pad soldered to an internal ground plane running through the whole board is enough, as the copper layer there spreads out the heat well enough to dissipate. It always amazes me that an outer-layer copper fill is not much better than an inner-layer one, so the larger coverage of the inner layer wins every time.

A great app note on getting started with thermal design from TI: https://www.ti.com/lit/an/snva183b/snva183b.pdf