If someone steals your watch, he can make large payments without any problems.
If you use your Apple Watch to make Apple Pay payments, you run the risk that a thief can use your Apple Watch to make payments from your account. He only needs to stay close to you when he has pickpocketed your Apple Watch. When you notice your Watch has been stolen, you will immediately take your iPhone and unlock it. If you set up 'Unlock with iPhone', the Watch will also unlock. The thief can then walk away, as long as he ensures that the Watch remains unlocked, and make payments with your Watch.
Tip: Switch off 'Unlock with iPhone'.