Since I have had a few readers of Beth thank me for teaching them a bit about soccer, I thought I would take advantage of a recent post on the Tifo Football YouTube channel (https://www.youtube.com/watch?v=h9QpXX47Mo4) to write here about the subject of that video.
The point the video makes is spot on, but I also wanted to give Tifo a shoutout for using an example from women's football (which is a rare occurrence in general soccer/football videos). The point of the video is that just because two players occupy the same named position, that does not necessarily mean that they have the same roles (term from the video).
Close readers of Beth will note that, though Rachel and Kim are both starting wing fullbacks on the high-school team (left and right, respectively), they have differing roles. While the primary tasks of the two are defense, Rachel has a secondary responsibility on offense, specifically making runs up the sideline to spread the offense. In fact, during the season, Rachel had two goals and an assist to Kim's single assist.
Another example from the high-school team are the two starting strikers, Rhee and Heather. Rhee is something of a big, bruiser of a striker. She can mix it up in heavy traffic and often come out the winner, while Heather, who might not weigh even 100 pounds, would do poorly in that role; she is more a finesse player.
My final example comes from the premier team, again involving Heather. Before her ankle injury, Lissa was the starting right wing and, though it's not specifically discussed in canon, she was there simply because she was too good not to start, but other positions were filled by others whose talents were more optimized for those positions. When Heather got a chance to show her skills because of Lissa's injury, she proved herself to be a better right wing than Lissa, thus relegating Lissa to being a backup.
In soccer/football, various players have differing skills and interests, and, all else being equal (which it rarely, if ever, is), the team that can maximize on the various skill and interest sets of its players -- that is, have the various positions and roles filled by players that excel in those SPECIFICALLY -- has a higher likelihood of winning the match.