By doing this, you create confidence within yourself because these are things you can be CONFIDENT you will execute as you are in full control of whether they happen or not.
The butterfly effect is very relevant in football. Mistiming a tackle by half a second, misplacing a pass to the wrong foot of your teammate, a shot one or two inches wide of the mark, can DRASTICALLY change the course of an entire match. Mentally, players must accept and embrace this fact in order to perform at their best. Instead of creating a burden of expectations, focusing on how a perfect performance should go, release those expectations and focus your thoughts on the "controllables".
It won't matter if you are an underdog coming up against FC Barcelona or Real Madrid, whether Carles Puyol is marking you or the man-child suspiciously too mature for your age group, you can enter any match full of confidence because no matter the opponent (or weather, or field condition), basing your performance on things like "staying on your toes" or "constantly checking your shoulder" are always within your control.