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“I have anxiously awaited the arrival of those such as yourselves. My names Megumin! My calling is that of an arch wizard, one who controls explosion magic, the strongest of all offensive magic! Do you, too, desire my forbidden strength, which is so almighty, I’ve been ostracized by the entire world?” – Megumin

1. My calling is that of an arch wizard (in bold above)




2. My name is that of Michael. No god am I, but one who serves.




3. A Barons title is that of a lord, but he may opt to be referred to as Baron (or Baroness) versus lord.




4. My job title is that of Head of Commercial Operations, however most of the team call me the Director.

In all of these sentences, why they use that of instead of just saying "A is B", such as "my calling is arch wizard"?

This is an effect of "heightened language". It is a deliberately fancy way of expressing something to mark the language of a strange character.

Here, the plain form would be "I am a wizard." You can raise the language a little by using the rare meaning of "calling"="job", to give "My calling is being a wizard." Then you can raise the language further to say "My calling is the calling of a wizard", but then avoid the repetition of "calling" by saying "My calling is that of a wizard".

Raised language is possible in most of the world languages, and Japanese has a particularly complex system of plain and raised language. This may be the translator's attempt to represent a complex and fancy expression of Japanese in English.

I'm not sure all of those are correct in everyday, terrestrial, non-fantasy English.

Both 2 and 4 sound much better with just a simple "is" instead of "is that of". Number 2 sounds particularly non-idiomatic. It should be "My name is Michael." Period.

4, also, should be "My job title is [perhaps in quotation marks or even italicized] "Head of Commercial Operations." because "Head of Commercial Operations" literally is his job title.

"That of" could be used to avoid saying plain "is" for some reason. Perhaps because the words following "is" aren't the literal completion of the statement.

For example:

"My name is that of my father and his father before him." You wouldn't say "My name is my father."

or

"My job title is that of the highest authority in the organization." because that isn't the exact wording of his job title.

Someone might also use "is that of" instead of just "is" in an effort to sound very formal, legalistic, or just plain pompous. Adding extra unnecessary words to your statements can help with that. It sounds to me like that might be the situation in example 1, where the author is trying to convey the personality type and social station of the speaker, Megumin.

The use of that of is correct in all instances quoted, except in the sentence, My name is that if Michael. I think poetic licence is being used here, meaning that in literature it is acceptable to deviate from the accepted use of the language, for effect.