The new iPhone models (iPhone X and later) have a new distinctive design—they do not have a traditional Home Button, which was typical for earlier iPhones. Tap to Wake feature sort of replicates that Home press function. However, it may also result in a lot of unnecessary screen waking.

How can Tap to Wake be handy?

I know you've landed on this page for a different reason, but let me highlight that this function might also be helpful if you cherish your data protection. Simply tap your iPhone's screen to check for the new app notifications instead of unlocking the device and exposing its content to the public. This is a no-brainer way to secure the information based on what you choose to show on the iPhone's lock screen. 

However, if you are determined to turn off this function, follow the steps below:

Settings -> General -> Accessibility -> Tap To Wake.

Eventually, this will bring you to the most convenient settings you need for your new iPhone at the moment :)