Tying helps the meat keep a neat shape while it's cooking and also encases any stuffing.
How to tie a butcher's knot
The easiest way to tie a joint of meat is to cut lengths of string and tie at 5cm intervals, trimming the excess string when you've finished.
Another way is to cut off a decent length of string, tie a knot and then loop the string around the meat, tightening it as you go. When you get to the end, flip the meat over then loop the string back over each section. Then turn the meat back over and tie it off where you started.
The most secure way to tie a meat joint is to start off by tying a knot at the end of the joint. Then make a loop in the string with your hand, and pull the loop around the far end of the joint. Pull the loop down to a 5cm interval from your original knot and tighten that off. Repeat the process. When you get to the end, cut yourself another length of string, and tie the joint off where you started.
Once the meat is tied, it's ready to roast.
More advice on cooking meat: