Some people refer to tires that are more resistant to punctures as "puncture proof" tires.
For example: Specialized All Condition Armadillo Tyre

Even though these tires are not really puncture proof, they do reduce a lot the chances of puncturing. So the advantage is obvious.

Apparently, they're not too expensive compared to regular tires. So, I was wondering why people would choose not to use these more resistant tires. What are their disadvantages?


Question: Understanding of Relays and associated circuitry
Answer: I remember from technician school that diodes are often placed on the coil of a relay or solenoid to prevent "bounce", which in the class, implied the coil might throw more than once when switched on. This is a subtle difference from what I'm reading in the other answers as voltage spikes. 

Given my lab and field experience, I would lean more towards the purpose of preventing voltage spikes with the intent of preserving the transistor.