Sadie, can I ask what promted the original question?
Is it because you can't find the twine, or for some other reason?
Like IC, I just use cotton twine I get at the home improvement store. It's a lot cheaper than "butcher's twine," works just as well, and is readily available.
I also agree with Kevin. Most of the time, recipes calling for trussing are an affectation. But when a dish really has to be trussed, it really has to be trussed.
For instance, properly prepared fish steaks will not hold their shape unless trussed. And, as others have pointed out, anything on a spit may require it to keep from flouncing around.
But if you're doing, say, a chicken in a rack in the roasting pan, trussing hardly serves any function.
For things like stuffed pork loin, it depends on how you want the final plating to look. If you truss, you get perfectly round slices, each of which is exactly the same as all the others. Really nice for a formal sit-down dinner. But if that isn't important to you, why go through the fuss?