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.

Understanding of Relays and associated circuitry
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?
