What To Look For In a Compressor Pedal?

What is the utility of a compressor pedal? Why do so many guitar players say it's their 'secret weapon' on their board? 

Compression is not easy to understand or hear. Sometimes it feels like it makes absolutely no difference to the sound, especially with lower settings on guitar. 

In this video, I am talking about what features to look for in a compressor pedal for the best sound possible, how I use it, and why you should or shouldn't get one.