Just a quick curious question, why is it important to have horizontal dropouts on a fixed or single speed bike?

And yes.. im a complete newcomer to cycling and wouldnt have know what the word "drop out" meant about 2 weeks ago, but yes...

I have Semi-Horizontal dropouts, it is possible for me to change my current bike to a singlespeed? or do I require/need a frame with dead horizontal dropouts?

Equally... would it also be possible with vertical dropouts??