["Blood is quite a nice look on you tho?" is maybe a bit too creepy to say out loud.]
It is an instrument to temporarily close blood vessels whilst performing surgery - they are not intended to be left in a wound until coagulation occurs. [So there.]
If you one day find yourself in a situation where you need to stitch a wound, and I am not there to do it for you, what are the chances that you will have access to proper tools? [Honestly, be grateful he isn't making you do this with a regular sewing needle.] If you can successfully stitch a wound despite such a heavy blood flow, you will be able to handle a crisis situation easily, should one occur in the future.
["Start in Hard Mode and advance from there" is the best teaching method, okay, that's Sebastian's story and he's sticking to it. He definitely didn't forget about that instrument's existence, that wouldn't be like him. Even if "stopping the blood flow" has never before been a thing Sebastian's had to concern himself with.]