Ex supply teacher here. I worked in a school that did an early finish on a Friday.
Pros, for the school they only had to pay me a half day on the Friday.
Cons, everyone knew which kids were on free school meals as the canteen made up a packed lunch for them to take home. I don't know it the canteen staff were paid less for the Friday.
It's useful if you have Muslim and/or Jewish children as they can go to a mosque or get home before the start of Shabbos. If you are in an area with a large Muslim population then you will find mosques running classes for children on Friday afternoon.
I'm fairly sure McDonalds made more money that day.
I'm sure for some parents it wasn't ideal. Having attempted to teach a 3-5pm class on a Friday afternoon I much preferred the earlier finish.
It was an interesting school, breakfast was compulsory and free for staff and students. You didn't have to eat, but you did have to take your class to the canteen.
Water, tea and coffee were all free and available at all times but you couldn't take the hot drinks away from the canteen.
Just to add to the great explanation from @wellington77 Sometimes things change fast in the subject you arte teaching. One of mine was, Health and Social Care.
So there is often new legislation, guidelines or something happens and is in the news that is relevant to something you are teaching eg a few years ago there was a Panorama programme about abuse in care homes, the students are learning about safeguarding so something like that might be included last minute.
It can also depend on the cohort, I love teaching equality and diversity. In Birmingham I would often have a class where I was the only white person and most of the class were multilingual.
That's an absolute gift because we can start with making a spider diagram of the diversity in the class.
The same subject when I taught on the Welsh border the class was entirely white, all English speaking but had done welsh as a second language at school.
We ended up doing a spider diagram of 'equality and diversity for the zombie apocalypse'.