I think this article explains URL encoding better than I can...
Percent-encoding - Wikipedia, the free encyclopedia
"When a character from the reserved set (a "reserved character") has special meaning (a "reserved purpose") in a certain context, and a URI scheme says that it is necessary to use that character for some other purpose, then the character must be percent-encoded."
Basically you encode reserved characters when they can be interpreted in the wrong context. I am by no means an expert in this area, but hopefully this example is correct and will make sense... Suppose the Center parameter required the use of ampersand before the coordinate value and didn't require a comma to separate (because the ampersand worked to separate). You might form the center parameter like this.
Center=&20&50
However, since ampersand is a reserved character that indicates a parameter on the URL, in order for the coordinates to be passed on and not interpreted as URL parameters, you'd need to encode it
Center=%2620%2650
where the %26 is the URL encoding of ampersand.
The article does say that where there isn't any confusion over context, you don't necessarily have to encode the character. But I'd say do it as a general rule...
