What is the use of thermal reliefs on vias for a copper pour?

1. Easier routing and fan-out of BGAs and other dense parts. Particularly when putting planes under the BGA.
2. Greater heat transfer to the planes on the PCB. You see this the most on QFNs and other packages with a ground pad on the bottom of the part in the center. This pad is intended to transfer heat to vias and then to the ground plane.
3. There is less chance for the via to get messed up due to plating, drill accuracy issues, or other PCB manufacturing problems (not a huge benefit, but a benefit nonetheless).

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