why would you raze a city?
A.) prevent it from culture flipping.
build your own city in it's place, since the new population is 100% your own and not foreign it's less likely to culture flip, since the people will have more loyalty to your own culture then the foreign culture.
B.) If you are specifically aiming for a Conquest victory over a Domination Victory. Going for Either you are garenteed to get at least 2/3 of the population, however if you don't raze any cities you are more likely to get 2/3 of the land mass granting you the Domination Victory over the Conquest victory.
C.) Just as a little bit of vengence against an AI that got ya mad. ie. they launch a surprise attack or raze one of your cities, you raze one of their cities in return.
Those are the only reasons I can think of