Building The Steven's Favorite

[Highpower]

Glocks don't have half cock notches because they don't have hammers. They use a striker fired system.

Another reason behind the half cock notch (secondary sear) is again safety. It serves to catch the hammer and prevent the gun from firing in case of an accidental hammer release. For example - if your thumb slips off of the hammer spur as you are pulling the hammer back to the full cock position. (Primary sear.)
As Tom mentioned it also serves to catch the hammer in case of a primary sear failure. (Dropping a fully cocked weapon and landing on the hammer, breaking the primary sear notch.

In the case of a half cock notch failure, there (normally) isn't enough momentum behind the hammer to fire the gun due to the short distance traveled to reach the firing pin.