Why did renaissance artists use a technique called one point perspective?

Renaissance artists used one point perspective in order to add a sense of realism and depth into their pieces. If you compare the pieces from the Renaissance to those of an earlier time you will see that they appear to have more to them than just a flat piece of paper. Perspective makes you feel as though you are almost going into the piece of work it adds realism to it. As an example take a look at a piece done during the Renaissance versus the Middle Ages you will see the one during the Middle Ages looks flat and less lifelike. But the one from the Renaissance seems like you could almost walk into it. 

What does that mean