When will you ever convert inches to miles? One of these is for measuring things that you can pick up with one hand, the other is for measuring traveling distances.
The useful thing about standard measurements is that base-12 is much more useful in daily life than base-10, which is only easier to work with on paper. Make a footlong object and you can very easily make 1/2 1/3 1/4 1/6 and 1/12 versions of it, make a decimeter long object and you can only easily do 1/2 1/5 and 1/10.