I have seen a diode and a resistor hooked up with a 12v pre-wired led. Now my question is how come?

Here is an example: http://www.phenoptix.com/shop/prewired-ultra-bright-blue-p-60.html

I know a good amount about leds and resistors, but nothing about this. Can someone enlighten me on this perhaps? I have tried searching, but didn't quite come up with much.

 

Many people either don't know how to calculate the resistors, and/or don't wish to spend the time to solder in a resistor and then add shrink tubing to insulate them properly.

 

The diode is so it will survive in reverse. Most LEDs would be killed by 12V in reverse.

 

reverse voltage is something generated by the user, in most cases? For example, someone attempting to hook it up backwards. What about in instances where it isn't user generated, I would assume this is possible. For example, a vehicle application, would this present itself there under normal operation or no? Sorry for the questions, I'm just trying to learn a thing or two now.

 

Mostly it is to make their parts idiot proof. You are not supposed to connect them backwards, but you can count on it happening more than once. The customer would come back and want replacements for free. This prevents this.

 

I have seen a diode and a resistor hooked up with a 12v pre-wired led. Now my question is how come?

Here is an example: http://www.phenoptix.com/shop/prewired-ultra-bright-blue-p-60.html

I know a good amount about leds and resistors, but nothing about this. Can someone enlighten me on this perhaps? I have tried searching, but didn't quite come up with much.

Not uncommon, I use a lot of similar ones on 24VAC with an additional series resistor.
If it doesn't work from 12 VDC just reverse the polarity.

 

They are idiot-proof LEDs, and sometimes plain convenient.

LEDs need to be current limited, which is what the resistor does, different value resistors for different input voltages.

LEDs are not build to withstand reverse breakdown, hence the extra diode to make sure the polarity is correct.

If you are 100% sure you have polarity and current correct from the driving circuit, you can save a lot of money by getting just the LEDs.