What if you get a new ruler that's 4 times as precise than the one you have that measures to 0.1cm? You don't want to record it as 0.70cm, because that's more precise than your measurement. But you could record it in 40ths with fractions.
Another way to look at it is that decimal is already a fractional system (1/10, 1/100, 1/1000) that doesn't allow you to use 90% of possible fractions.