All measurements in the real world come with error bars. Indeed, at a certain scale the thing you're measuring generally becomes ill-defined. The length of a piece of string to tenths of an inch is pretty easy to define. If you magnify it so the end is rough, it's harder. Zoom in to subatomic, and things get really difficult to define, let alone measure.
Given that any error bar contains some rational numbers, the case can be made that rationals are all you'll ever need for measurements in the real world. Nothing can ever be measured so precisely that it has a definitively irrational length.
However, irrational numbers do serve as a useful abstraction when dealing with the real world, since one is often working in different scales. A rational approximation of pi that works in one context -- say, construction paper projects -- might not be good enough in another -- say, orbital mechanics. Identifying the abstract, transcendental value has the practical application that you can use a rational approximation appropriate to the scale.