Empire: Total War Message Board  

I'm going to go out on a limb here, but it's likely for people that:

A) Don't or Can't use the trainer
B) People that want to remove the fog of war, but only that.
C) Perhaps they prefer an "internal" method of achieving that goal, rather than through external means.

Often, I very much prefer a modification to a game than a trainer, albeit that a trainer is via another person's work and thus saves me the time, a mod is "always on" and can potentially do things a trainer can not. It really depends upon the desired effect and how plausible it is to alter it in memory once it's there, or to simply have it put into memory how you wanted it to begin with.