Should have said that the #define and ifdef is an MPASM directive and not to be confused with mnemonics.
Aside from the better readability of using #define for port pins and/or pin values, they are useful in making procedures and functions more versatile. Say for instance, you have an RGB led, or 7seg display routine set up for common anode. It is easy to expand those routines for common cathode, simply by using the #define, ifdef, or ifndef.
Code:
#define ComCathode
...
...
...
If red_duty > duty then
#ifdef ComCathode
red = 1
else
red = 0
#endif
else
#ifdef ComCathode
red = 0
else
red = 1
#endif
end if