Decimal digit property - What's it for?

[Arcane Jill]

Hi,

It has been explained to me that the "decimal digit" property has the 
following meaning: "Decimal numbers are those using in decimal-radix 
number systems. In particular, the sequence of the ONE character 
followed by the TWO character is interpreted as having the value of twelve".

What's the point of such a property?

I mean, it's quite clearly ignored in sentences like "My phone number is 
0044-1727-6000000", or "The codepoint of the space character is U+0020".

What possible use could any mechanical algorithm make of the "decimal 
digit" property that it could not equally well make of the "digit" or 
"numeric" properties?

Jill