For years I considered a micrometer or caliper a bit of a luxury. Then I found this one on a tool stall (probably Squires) for under a tenner and though, "Well it might come in handy".
How right I was. To be honest once I'd got over the initial stage of playing with it, finding out the size of everything within arms reach, the tool went back in it's box and didn't see the light of day for a while. Gradually though, I found some real uses for it - picking the right plasticard for one.
With a kit like the C15, it's pretty much essential. Lots of thicknesses of wire are supplied but if you want to find out which one to use where then you need a method of measuring them. Either that or get good at differentiating between 0.9mm and 0.7mm by eye.