Fractions are just numbers, but inches are measurements. To do any sort of conversion to inches you need to change fractions to fractions of some measurement. Let's say, for example, you need to convert fractions of a foot into inches. Let's say the fraction is one third. Then we need to know what one third of a foot is in inches. We need to know how many inches there are in one foot: 12 inches.
Now we can find how many inches there are in one third of a foot: ⅓×12=12/3=4 inches.
So, to do conversions involving measurements you do need to know how to compare different types of measurements before you can work out the fraction of one measurment in terms of another. That is, how many inches in a foot or yard or mile or whatever.
You might also see questions that ask how many seconds there are in some given fraction of an hour. It's the same idea: how many seconds in an hour, or how many seconds in a minute and how many minutes in an hour; and so on.