Some weeks ago I wrote a post about Avoiding Tunnel Vision when writing a novel. (You can read it here). This post is related to that and takes its name from TvTropes. The core of the trope is as follows (taken from the trope's page itself as seen here).
"The writers give the impression that this character is having just as many adventures as the hero, only offscreen. In other words, they're the Hero of Another Story."
The test of this trope is asking if this person could be The Protagonist instead of the one you have chosen and saying 'yes'.
This trope is a useful tool in avoiding Tunnel Vision because it helps break the idea that the hero is the only hero. There are other heroes and they are having other adventures in other places. Simply alluding to these places and adventures will develop the world of the main characters. Adding one of them will force you to think about what is happening outside the main characters' sight and you will know more about the plot as a whole. All of this will enhance the main narrative because you have created the Full Picture.
From a Watsonian POV, The Hero of Another Story is also great for the helping the reader maintain their Willing Suspension of Disbelief.
1. You don't have to force your reader to accept that the Rag Tag Bunch of Misfits can defeat the Evil Empire all by themselves because there are OTHER heroes working towards the cause.
2. The team can be modified as needed to suit a story arc by slipping in one member or another. A Six Ranger can drop in to help out on this particular mission and then go do something else that's necessary to advance the cause (and by this, advance the plot). By contrast, one or more members of the group can go help that person while the remaining heroes focus on the events that the reader will see. If this is done well then it will give the story a broader scope and a wider world, but if you're not careful it will become Put On A Bus.
3. They can arrive to save the main heroes and it won't (if handled appropriately) appear like a deus ex machina because the reader knows that they are out there and that they are just as competent as the heroes themselves.
My own experience with this trope is why I recommend it so highly.
Part of the pre-writing for A Mage's Power involved filling the guild Eric would join. The Dragon's Lair is a mercenary guild so I thought it would be odd if there weren't other mercenaries going on missions and making money so I created a list; their names and abilities and personalities. This provided me with a pool of characters to draw from and allowed me to avoid the idea that no one else was doing anything. They gave the preception that there are mercenaries in this organization that have lives outside of their relation to Eric and roles in the plot that do not connect directly to Eric. Furthermore they gave the organization character and life. Instead of a plot device for building Eric's confidence and magical power, it is its own place and Eric happens to be part of it. I wish I could have done more with them which leads to my final point: Fanfic Fuel.
I got my start in writing fiction from writing fan fiction. I spotted a potential in the canon of the story I watched and used that to develop my own story within that canon. It was a lot of fun. My point is that readers love fanfic fuel. After finishing a story, my first thought is to look up fanfics to prolong my enjoyment of the story and see what others have come up with. By including a Hero Of Another Story you provide them with a wealth of possible adventures just waiting to be written.