Semantic versioning. 4.0 -> 5.0 indicates that the new society may be backwards-incompatible, whereas 5.0->5.1 implies that society will keep all of its existing APIs while adding more. A patch version that changed its internals without adding or removing any new features would be called 5.0.1, but society versioning doesn't include the last digit because civil servants don't ever get credit for their behind-the-scenes improvements.

I think it gives it a clearer indication that it's a version number, whereas "Society 5" either sounds like either yet another sequel to Society, or part of the title itself.

Also, point-oh sounds cool in English and just based on how much they use fractional percentages in Neon Genesis Evangelion, I assume it sounds cool in Japanese too.

It disambiguates it as a version number, as opposed to a count or size. I agree it sounds dumb, but you do immediately know what it means.

I’ve seen a surprising amount of things with functional version numbers.

The frozen vegetable “meat” that’s being delivered my way is on v1.1 apparently. If people get used to food versionning, I think they’ll be fine with any other versionning.

Sounds more technical this way, and avoids confusions with "Society Five" sounding like a group of people.

Do you remember Web 2.0?

Significant figures. Without that decimal place we don’t know how specific the measurement is