I'm trying to figure out the purpose of some of the tools out there, and when one would need one tool and not others, and vice versa
Looking at the Knipex pliers wrench, which looks like a combo of pliers and wrench in one. As the name implies. Other than the portability factor of having two tools in one, is there much of a difference (e.g. performance-wise, etc.) of just getting the combo wrench and the Knipex cobra pliers? Are the plier wrenches truly needed if I already have the combo wrench and Cobras?
Same thing goes with the diagonal cutters vs mini bolt cutters vs. wire cutters
I wouldn't use my pliers-wrench instead of pliers. I only use the pliers-wrench when I need those perfectly parallel jaws. I also baby them, because I love them so much, so I wouldn't want to chance things slipping in the jaws, possibly dinging them up or creating pits. They'd probably stand up to that, but I would never want to grip the threads of a bolt with them. I would grip the head of the bolt with them instead. You know what I'm saying?
No, they're not
needed, if you already have every size metric and standard open ended wrenches (and the sizes in between those, and smaller), but they sure are nice and would compliment your kit nicely.
If by "combo wrench" you're talking about an adjustable wrench, then yes, the pliers-wrench is needed.
If I were starting over, I'd probably skip buying some of the larger sized combi wrenches and just use a pliers-wrench to cover everything above 1", let's say. You need clearance for your hand gripping the pliers-wrench's handles, as you turn a fastener to use them, so they're better for wide open spaces. That angled box-end on a combination wrench has saved me countless times. Just barely getting the wrench into some confined area, and getting that 12-point onto the bolt-head, in the dark, on a Wednesday, and being at the perfect angle to just barely get a hold of the end of that long-pattern wrench, happens a lot.
The pliers wrench is great for portability, keeping one in your carpenter pant's side-pocket, to walk around and tighten various sized fasteners, without worrying if it's metric or SAE, such as setting up a lighting truss, for instance. So fast, with what they call its ratcheting capability, where you just loosen your grip to re-set the tool on the bolt-head, is worlds better than readjusting the thumb-wheel of a crescent wranch.
Side cutters, dykes, bolt-cutters differ by their leverage capabilities, and sometimes I wonder if jaw-hardness and metallurgy factors into it, and I'm sure it does. Cutting speaker wire is handled well by some small dykes, but there are all kinds for different types of duty.
