Why is the pasta often cooked in the sauce in al'arrabiata recipes? I looked up some recipes for pasta al'arrabiata, and one part I find a bit confusing is that most of them finish cooking the pasta in the sauce. So you cook the pasta separately until it's almost done, and then mix it with the sauce and finish cooking. I'm used to simply cooking the pasta and the sauce each for themselves, and only then mix them.

I'm an inexperienced cook, and this part adds some complications where I'm wondering if they're worth it or if I should simply cook the pasta and the sauce separately as usual. I'm comfortable with cooking the pasta alone to something resembling al dente, but mixing them both just before they're finished and then cooking both together makes this more difficult. I also usually use a rather small pot for the sauce, so I'd have to adapt that as well.

So, I'm wondering what the purpose of this step is, and whether I should just cook sauce and pasta entirely separately.
While Italian cuisine is defined regionally, the majority of Italian pasta dishes have one cook the pasta until almost done, then finish in the condiment. While there are exceptions that include thick, long cooked sauces like Bolognese or sugos, finishing in the condiment serves several purposes: (a) the pasta and the condiment can be combined, (b) the pasta finishes cooking, (c) it absorbs the flavor of the condiment, and (d) the condiment (sauce) thickens and emulsifies from the addition of the starchy pasta cooking water (this can be controlled by adding cooking water and/or allowing it to cook off). Yes, it is more difficult than cooking separately, and it can be a little nuanced, but it is not very technical.  With a bit of practice, I think you will find this step worth it.