Other than one piece of string tied at the narrow end to hold the tucked-under pointy tip, making the whole roast more-or-less evenly shaped, I don’t see what the point is of the rest of the string? What good does the piece of string around the center of a beef tenderloin do?
Prevent it from escaping? I got nothing here…
It is to make it evenly-shaped, so that parts of it aren’t underdone while other parts overdone. It holds the whole thing together cohesively, which is why you need it on the whole thing. And if you have leftovers, may I suggest making Beef Stroganoff with them?
I should clarify - the question is only in the case where the tenderloin is NOT first butterflied, stuffed and rolled. If it has already been stuffed/rolled then it’s clear why you’d tied it.
But, in this video (for example) Melissa Clark ties up a roast beef loin without first slicing it apart. I don’t see the point (other than at the narrow folded over end). Is there one?
Alice the Goon covered it. In fact I tied one today. It cooks evenly and provides for a nicer display on the table.
An untied tenderloin will be oval shaped coming out of the oven whereas a tied one will be round. It’s purely a presentation issue.
Here is Cook’s Illustrated Guide to Trimming and Tying Meat and Poultry.
They say:
Tying cuts of meat keeps them compact and ensures even cooking.
They offer additional reasons in the text.