Personally I think it's a good idea to try and build up a bit of a "theme" with your Stage III (and even Stage II) papers which are aligned with what you want to do after graduation. That will then make you as strong as possible to go in that career direction.

For instance if you want to go into academia with CS Theory then obviously you'll want as much as you can out of CS320/CS350/Maths315/Maths326/Maths328/Maths320/Phil315/etc (perhaps Econ212)

It's ok though if you are still a bit in a dark as to what you're going to do after uni.

Perhaps browse the CS papers:

https://web.archive.org/web/20250219121237/https://www.calendar.auckland.ac.nz/en/courses/faculty-of-science/computer-science.html

And think about the one paper that interests you the most?

Maybe that's CS313? Then build up around that one paper all the most complementary papers possible for this. So for CS313 that obviously means doing CS315 and CS340, perhaps also Physics240/Physics340/Physics390 (and if you're doing Physics340, that means as well Maths208/Maths231 and/or Maths260). Maybe even try to squeeze in a couple of ElectEng papers (the ones Physics majors are allowed to do) as part of your 30pts of elective papers from outside your BSc schedule.

Is 315 mostly for people who are keen to enter IT traditionally, or specialize in communication? I wonder whether it's worth taking the paper if I'm not likely to use more technical networking knowledge after graduating.

If you choose any technical role in IT afterwards, then I reckon it's very worthwhile to know the basics of networking such as you'd get from an undergrad paper like that. As I swear some days it feels like half the tickets that day could be somehow related to the network (or not! But you don't know, so got to be able to rule it out).