A retrospective is part of the Agile development methodology used at a lot of Tech companies where a team looks back and reflects at regular intervals to identify the things that went well for the team during that period, the things that didn't go so well and can be improved upon, and the things that the team currently isn't doing but ought to be doing.

Any sort of a process (a process for writing, for learning a new course, for getting better at playing football, etc) is incomplete without such regular retrospectives. 

Because, without this, we would be like the monkeys in a cage. To recap the monkeys in a cage story:

A researcher puts five monkeys in a cage. There’s a bunch of bananas hanging from a string, with a ladder leading to the bananas. When the first monkey goes for the bananas, the researcher sprays all five monkeys with freezing water for five minutes. Some time later, when a second monkey inevitably tries to go for the bananas, the researcher once again sprays all five monkeys with the cold water for five minutes. The researcher then puts the hose away and never touches it again. But, when a third monkey tries to go for the bananas, the other four attack him to prevent him from climbing that ladder. They are afraid of the punishment that may come.

Then, the researcher replaces one of the monkeys with a new monkey who wasn’t part of the original experiment and was never sprayed with water. And, as soon as he touches the ladder to go for the bananas, the other four monkeys attack him to keep him from doing so.  If he tries again, they attack him again. Thus, the new monkey learns not to go after the bananas because he’ll get attacked if he does.

The researcher replaces a second monkey with another new monkey. When this monkey goes for the bananas, the other four attack him, including the new monkey who was never sprayed with water. The researcher then continues to replace all the monkeys one at a time, until all five of the original monkeys are removed from the cage. Each time the newcomer goes for the bananas, the others attack, even when they, as new monkeys, have never received punishment for going after the bananas. And thus, the new monkeys, who have never been sprayed with cold water, learn not to go after the temptation of the bananas.

The researchers hypothesize that, if they were to ask the monkeys why they don’t go for the bananas, they’d answer “because that’s the way it’s always been done”.

When we don't reflect on our process, we stop questioning our process and start to simply follow the process for the sake of following the process. Because that's how we've always done it, or because that's what we're supposed to do.