Basically CHF_BEELINE would simply be a nice alternative to monsters simply zigzagging everywhere (which is possibly part of the reason they're pretty hopeless in pathing)
More or less, unless the player is at a different "horizontal coordinate" (i.e not straight ahead) they will move straight towards the player/target without constantly changing angles (unless they need to)
A feature such as this would be very useful, especially in allowing for better monster pathing/intelligence.