Groomers call that a 'pee string'. ha. It actually helps to direct the flow of urine away from the body, so it does have a purpose. If it's cut all the way back, the urine will just spray out and get on his tummy, legs, etc. So definitely leave it. A little trim is fine. A little pee string is MUCH more sanitary than having pee all over his underside and getting on his legs.
"Having heard all of this, you may choose to look the other way.....
but you can never say again that you did not know." William Wilberforce 1759-1833