I'm not usually a rule basher - I'm a little bit pedantic about most rules, but I just don't get this...

There is no equivalent rule that I have heard of in design (ok maybe in calligraphy, but you can also have circular lines to write on) so I don't understand why having a straight horizon makes a better picture?

- if you are in choppy waves and want to convey your 'queasy' would it make sense to take something for disorienting purposes?
- If you are looking up squinting with your head at an angle and something resonates with you and you want to take a photo of that, why straighten it?

I'm sure I could think of more situations that I would like clarification on

In design, it's more like, if you want it skew, make sure it's obvious that it is skew, then someone knows that you did on purpose that you were just not doing it because you forgot or by accident.

So can someone (try to) enlighten me as to why this is some to strive for from an aesthetic purpose?
It may not make me change my opinion, but I am interested in possibly hearing something I may not have thought of or do not know?
Ta