People use the curly 8mm line because it comes in the kits. It's a pita with pots & short 3mm lines but probably wouldn't be as bad in beds where the curl can be laid flat.
Irrigation systems & hardware are also a whole new world for most of us. Internet shopping doesn't help, either, because we can't touch it & we've never seen it working in person, either. So we start from scratch, build on experience until we get something that works for us.
Ya well if you are on this forum reading this thread then there is no reason to buy kits with stiff 8mm lines and fitting you don't need. It's not cheaper to buy a kit. You'll end up with a better system by going to sustainablevillage and buying sensors for $6.90 each, adapters (barbs) 3mm to 1/4"-65 cents and enough 3mm line to hook them up to some cheap 1/2" black hydro line. 3' of 3mm for every sensor. 1 or 2 through hull 1/2" adapters for gravity feed res or some more 1/2 fittings for a pressure reducer. Nobody that can read this thread should get stuck with that worthless 8mm line and all them fittings that come in kits.