Why, when providing FM radio designs, do people use physical parameters instead of electrical? I.E. 2 turns, 20awg, 2cm dia.
Often times these plans for receivers used lower value coils that most parts houses wouldn't carry. This meant that the user would need to make them himself. About 99% of the time, the user didn't even know what inductance was, so specifying mechanical specs was easier for the user to interpret, and gave them an idea of what they needed.
About 5 years ago (age 11) I got my first soldering iron and a kit like this. At that point I had no inkling as to what inductance was, and just barely understood DC resistance and the effect of capacitors on a DC source (as well as how various components behave in parallel/series but that's off topic). Even if I did know, I would have needed to buy more equipment than I would know what to do with in order to test to make sure I got the right value. At that point, mechanical specs were simply simpler for me.
In short, KISS. Keep It Simple SnapCircuits!
Many readers do not have the skill set to implement design criterion into physical reality. When I first started out, I was so appreciative of this kind of information. I simply didn't have the skills to apply mathematical information into physical implementation. Also, having a KNOWN solution for the circuit gave me the confidence to move forward. Some "Guru" who says use this gauge and this number of turns helped me to make the circuit to work. I know my answer here is short and sweet, but in reality when providing information to others, you do need to take into account that you might be speaking to an audience from beginner hobbyist all the way to a PhD. Those who DO understand have a starting point.