Hi (yet again!)

Rendering is the process by which your edited footage is transformed to whatever is your final destination format.  You have to use it simply because without it, it is highly unlikely you would be able to "show" the fruits of your hard work to anyone who does not have access to your computer.

Preview Rendering is the process by which MEP creates a "temporary" version of all or any selected part of your footage, especially one which contains any complex transitions or other effects which, without such rendering, would not play back smoothly on your computer.  IOW it allows you to check that what you want to do actually works in real time.

HTH

Jeff