A dry lab session is a good training for some technical steps of the procedure. The pursestring suture implicates a good or bad start of the procedure and during cadaver sessions or proctorship even this is a step that requires a learning curve. Therefore it is very helpfull to practice on a model while getting feed back of an expert. It's also a good practice to get used to a limited working space.
Set up for practicing a purse string suture
Both time needed to perform a pursestring suture and quality of the pursestring can be judged.
Gap between stitches and therefore suboptimal sealing of the lumen