I wonder about students whose teachers scold them for writing in the first person, and the same teachers turn around and give the students personal accounts, journals, or even first-person fiction stories and novels that are not brought down with the same scrutiny. Suddenly, the rules that govern writing, or rather the student's understanding of them, become convoluted and inconsistent. It leaves students confused and frustrated.
Welcome to genre.
Without an understanding of genre, all writing is the same and anything goes. The result can be messy. Objective research with out-of-place anecdotes, awkward personal essays that never really say anything personal, and even job applications that give a fine description of the job but say nothing about the applicant. If you don't understand the rules of the genre, then you'll probably do it wrong.
Genre shapes the rules of writing. It is a complex topic and we don't really get much exposure to it outside of fiction. When we read a science fiction story or watch a science fiction film, we have certain expectations. The same applies when dealing with romance, fantasy, tragedy, comedy, etc.. Different genres have different rules and it's important to follow those rules.
For example. You're watching Lord of the Rings and Gandolf, in all of his trope-setting, magic casting, staff waving glory...takes out an energy weapon. Like a light saber: a mechanical apparatus that uses sophisticated technology to develop and release concentrated energy.
Out of place, much? I'm not saying it wouldn't be awesome. I'm just saying that, in a world with magical rings, dragons, and immortal races, an energy weapon is out of place. When we engage in a fantasy story, we expect fantasy.
Genres outside of fiction operate the same way. Based on the kind of reading we're doing, we'll have different expectations. If you pick up a detective novel, you'll expect mystery, intrigue, red herrings, and a protagonist who says less than he knows until the end. If you read something more personal, like the author's preface, you'll likely find the author referring to their writing process, their emotions – something more personal.
The same issue with expectations applies in all writing. The aforementioned personal essay will deal with personal experiences, beliefs, and give the reader a personal perspective on the author. It isn’t the best time to explore serious scientific issues and research: that's what research essays are for.
Bear in mind, however, the rules aren't arbitrary. Like fiction genres, they have developed over time to fit specific needs and purposes. Therefore, writing for a genre, whether of your own volition or because you're required to do so, you are fulfilling a specific purpose. The rules in question govern the way to structure a thesis, argument, organization, style – almost anything and everything.