I grill a lot of poultry and stuffed meats on the grill using the rotisserie. If I don't truss the chicken it will flop around and throw off the balance. The presentation would be terrible. Butterflying a piece of beef or pork and then using a filling, rolling up the meat, it needs to be tied to hold its shape.
I have at times bought a turkey roast that comes in a mesh type containment device for roasting. I hate them things. You roast it up beautifully and then have to ruin the presentation by removing the mesh.

RE: "The presentation would be terrible. "

Yep- these are all the supposed logical reasons for trussing. And many people do. But even in my professional cooking days I seldom trussed and my product results, nor the plate presentation seemed to suffer at all.

In commercial kitchens where I did much volume cooking, trussing became kind of an impractical pain in the behind- so I seldom did it. Things always seemed to work out fine, though... even stuffed loin of pork, or stuffed butterflied boneless lamb legs for 250!

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?