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It's a number associated with a unit. Making it a distinct type, even when the underlying representation is a float, makes it harder to make mistakes by substituting or combining units inappropriately.

This sort of stuff works really well when the type system incorporates units of measure directly (usually as some kind of generic type parameter). F# is a good example.

This is similar to having 3D position and direction in different types (e.g. point and vector instead of a generic 4-component vector). It looks nice in theory, and it is 'type safe' (e.g. you can't add 2 points, only point+vector or vector+vector), but writing real-world code with this is awkward.

Some languages do include "natural" in their numeric tower.