A crucial skill for all drivers
It is possible that only your peripheral vision will spot something moving,
such as a car door opening, a child running out between a car,
a pedestrian crossing at the last second.
The area to begin planning for making a turn, or to deal with traffic lights etc.
The area which in the least, you should be considering looking ahead whilst driving to plan
for the things coming up and how you will deal with them.
What are the cars ahead doing?
Which direction does the road go?
Are there traffic lights or junctions ahead?
Possibility to plan for the MSPSGL routine if you see the end of the road, a turning you might want.
A bus pulled in that might move off at any moment.
The reality of not building up this skill, is that you WILL miss things. If you constantly only look at the rear of the vehicle in front of you, you are going to miss everything that is happening in front of that vehicle. You'll probably become a constant hard breaker because you are dealing with everything at the last second.
Likewise if you only ever look within the Early Planning Area you are going to miss everything that falls within the Instant Danger Area, at the could easily be a pedestrian stepping out in between some parked vehicles.
By keeping your eyes moving up and down the road, you will be filling your brain with NECESSARY information.
Over time your brain works out what of that necessary information is "important" and what is not.
This skill then grows into something that filters out unimportant things but allows you to remain fully aware of the important stuff.
Our brains are computers, and with proper training in particular areas, become SUPER computers to deal with endless amounts of moving information such as driving along a road.
If you don't train your brain to become a super computer it won't work like one.
Although you don't spot it yourself, if you asked someone to watch your eyes as you scan up and down the road, your eyes will marginally wiggle from side to side as well on the upward and downward journey, so that you are effectively SCANNING the road.