A floor pump up to 8 bar generally has a larger barrel volume and therefore delivers more air per stroke. It is particularly suitable for high-volume tyres, such as those on MTB or trekking bikes.
A floor pump up to 12 bar has a narrower barrel. This makes it easier to pump in the higher pressure range, although more strokes are required. This is especially useful for road bike tyres.
You can find more information about choosing the right floor pump in this blog.