Thanks for the question. I have added this to the blog queue - it does bear some explanation.
Throughout each play, decisions are made by the game engine as to what happens on the field, using the obvious factors such as skill, fatigue, etc. One of the variables that are used is how familiar that player is with the play they are performing - so the more familiar your players are with the plays in your playbook, the better they will perform them. There is also an influence from how familiar they are with the opposing team's play. As they see a play during a game, they increase in their familiarity of it, but the best way to increase their familiarity is to practice against it during the week. This is what the scout plays are - they are plays from your upcoming opponent's playbook that you choose to study and practice against.
On the screen they are listed with the play most frequently used by your upcoming opponent at the top. If you Rex the scout plays, it will choose the top 5 plays, but you might want to not bother with plays that your team is more familiar with or plays that haven't fared well for the opponent.
Does that help explain it?