Yeah... having more than 5 friends nets you a whopping extra 5 feathers a day. So that's more than half a month to save up so you can raise a 2* into a 3*. YAY! the incentive is just so attractive, right?
On a serious note, you're right. In other games, like FFBE, whoever you set as leader can be used once every 3 hours in addition to your normal members of a team to defeat a map or challenge.
"GANTZ"'s Noctis is basically the reason I was able to obtain and max out the Arianna Grande character, even though I had only started playing the game right at the beginning of the event.
In MKX, after a match goes on long enough, you can summon a friend's leader character to do their highest damaging attack once per match. They used to do their full, real damage, but that shit got nerfed after a while. But at the same time, whether you used them or not in the battle, choosing an ally each battle gave you points that you could accumulate and turn in for a single card that could turn out to be another low tier character, an equipable item, or free attack upgrade or level up... gah, it definitely has its rewards to have friends and use them. Hell, if you used an ally, they got 100 points. If an ally used your champ, you got 50.
With FE... You get close to nothing for having or adding friends. Hell, in FFBE you could send you friend small little bits of stuff for free, and it would be stuff your friend set themselves to receive. It would be nice if for every day you were one, you could have an active individual friend send you some feathers, or shards, or crystals, or an orb (or even pieces to collect to make whole orbs). Seriously, they added in this feature and never utilized it except for really bad randomizing schemes.
that being said... Enjoy my Kagero, reprobates. She's been set as my lead unit for a while now. Handled with care, she puts in WORK. Your dragon has falchion problems? Kagero has an answer for that. Your armored units don't like that mage character? Kagero has an answer for that. You don't like that ninja/assassin debuffing you hardcore? Kagero has an answer for that. YOU'RE WELCOME. She's only my favorite and best draw ever since the beginning of the game.