RTF is a global modification that replaces all towns in the game.

the thing is, a post in the city replacement mod topic – many won't use it because it means losing the standard creatures.

This is a debatable point – the fact is that NCF creatures will only participate in the game if they are in a special package (e.g., a megapack), and even then, only as neutrals. The coolest thing about NCF is the alternative upgrades from a Fortress to a Hill, they can really affect the game, while other neutrals are just dummies for gaining experience.

Replacing a city gives you the opportunity to play as a new faction with new units, or to remove a faction you don't like from the game.
For me, replacing a faction gives me the opportunity to develop ideas that cannot be developed in NCF, to create heroes, to come up with a city development principle, and to play with my own creatures properly, rather than with pre-set armies.