Smaller, lower power, more reliable + a somewhat open spec being developed around it (unlike things like homeplug which are kinda murky). Generally it's the same people who developed the ethernet spec that are developing single pair ethernet, so it's designed to play nicely with it.

Cheaper cabling, at least in theory, but I'm not sure ethernet (for data) is the right comparison.

IMHO, in the near future, data-intensive connections will move to wifi 6/7/etc, or to fiber. Copper will be for providing data and power. IMHO, it makes more sense to compare SPE to current PoE networks/devices, powered wifi devices, CAN bus, rs485/modbus, and other wired IoT and industrial networks.

Well my house has electricity sockets but no ethernet sockets, so it would be less expensive than getting those installed. If I could stomach sub-wifi speeds.