Because that's the entire point of college. It's supposed to teach you the fundamentals - how to think, how to problem solve, how to form mental models and adapt them, how things you use actually work. Knowing how different sorting functions work and what the tradeoffs are allows you to pick the best sorting function for your data and hardware. If the tools you have aren't doing the job, you can mend them or build new tools.

So you know which sort to call because there isn't a right answer for all cases.

And so you can write your own because you're probably going to want to sort data in a specific way. Sort doesn't mean in numerical increasing or decreasing order, it means whatever order you want. You're sorting far more often than you're calling the sort function.

The problem is that a computer science degree isn't the right training for most software engineering jobs.

My high school computer science teacher (best one I ever had) once told us this anecdote when we were learning sorting algorithms:

He was brought in by the state to do some coaching for existing software devs back in the 90s. When he was going over the various different basic algorithms (insertion sort, selection sort, etc.) one of the devs in the back of the class piped up with, "why are you wasting our time? C++ has qsort built in."

When you're processing millions of records, many of which are probably already sorted, using an insertion sort to put a few new records into a sorted list, or using selection sort to grab the few records you need to the front of the queue, is going to be an order of magnitude faster than just calling qsort every time.

Turned out he worked for department of revenue. So my teacher roasted him with "oh, so you're the reason it takes us so long to get our tax returns back."

Thinking that you can just scoot by using the built-in version is how we get to the horrible state of optimization that we're in. Software has gotten slow because devs have gotten lazy and don't bother to understand the basics of programming anymore. We should be running a machine shop, not trying to build a jet engine out of Lego.

I mean, the lesson I got from my 10X class was pretty much that: "never write your own math library, unless you're working on maintaining one yourself".

funnily enough, this wasn't limited to contributing to some popular OS initiative. You can call YAGNI, but many companies do in fact have their own libraries to maintain internally. So it comes up more than you expect.

On a higher level, the time I took to implement a bunch of sorts helped me be able to read the docs for sort(), realize it's a quicksort implentation, and make judgements like

1. yeah, that works

2. this is overkill for my small dataset, I'll just whip up basic bubblesort

3. oh, there's multiple sort API's and some sorts are in-place. I'll use this one

4. This is an important operation and I need a more robust sorting library. I'll explain it to the team with XYZ

The reasoning was the important lesson, not the ability to know what sorting is.

> why am I implementing something if I just need to understand what it does?

So you can pass job interviews, of course!