Imagine if the Watch let you do this without a passcode. Then your watch was either swiped off your arm or it fell off somewhere and you lost it. Hypothetical. Then someone could take your watch and make Apple Pay purchases while it is connected to Wi-Fi, without any authentication. Or, someone could access all of the data on it, your emails, texts, phone calls, contacts, health data, Medical ID, etc...all because you were too lazy to put a passcode on it (thus causing the data to be encrypted and protected by your passcode).
Don't be lazy. Stupid negligence like this is how identify theft happens. You don't seem to understand the security implications of trying to use a device marketed as the "most personal device ever" without any kind of security protection.