If the existing key is in good condition this is fine but if the original is worn then you are reproducing the wear onto the copy. This is one reason why copies sometimes don't work well. In fact a copy will never be as good as an original, so a copy of a worn key is an even worse worn key.

Cutting to code creates a new key -- a totally brand new key. If the locks are in good condition the code-cut key will continue to operate them for many years, and additionally this key could be used to make good quality copies.