Cordage is one of those things you could make in the field, at least theoretically and provided you found the right plants. But, honestly, it is so much easier to just pack some in each of your survival kits.
There are just so many tasks that are made infinitely easier with even a shoelace, let alone several feet of good quality paracord.
–Lashing together an expedient shelter.
–Hanging food from a tree to keep it away from animals.
–Tying gear to your pack.
–Replacing broken boot laces.
–Bow drill for starting fires.
What I think is one of the coolest features of paracord is that in spite of the incredible strength, it is much thinner than you might expect. Seriously, I have shoelaces that are thicker. What this means is you can easily fit a dozen feet or more in even a very small survival kit.
Coupled with having cordage is knowing how to use it effectively. I highly encourage preppers to head to their local library and find a book or two on knots, then practice some of the more useful ones like the square knot, the bowline, and the clove hitch.