I don't know what you're talking about regarding that it as been proven many times, a lot of people get into characters because of their backstory, which can be showcased in a story mode, in addition to their gameplan, so a a story is important for that reason as well as other reasons like why the world of this game is worth investing. People can also pick a char based part of their character design, overall looks and personality, which can sometimes relate to that character's story. One of the reasons I like Reptile in addition to his gameplan and him being the green palliate swap Ninja is that he changed his overall look overtime in the games, from the human Ninja until the hybrid of human Ninja and lizard man that he is now, which reminds me of how The Undertaker has changed his look overtime in his wrestling career, and that part of Reptile is connected to his backstory as he came to a mental state that prevented him to keep his human from because of the separation of his race of Zaterra, that is explained in the MKDA story. A backstory is also the main reason why I like Scorpion as much, because he is the undead Ninja of the thirst for vengeance, again in relation to The Undertaker, so his backstory is the pretty much the main reason why I like Scorpion. And yes I also don't play as Scorpion as much as I play Reptile because his gameplan in generaldoesn't fit to me like Reptile's, but I like both characters for their backstory, personality and character designs, with Reptile also having the general gameplan. I always go for character's look, personality and story in addition to the gameplan. If it's just the gameplan alone while all the other areas of character backstory, design, looks and personality don't appeal to me, I won't play that char.
Story modes help you get a connection with a character that you eventually like and then play, and they also help you understand the overall feel of the universe the game takes place and it also helps you understand why the chars even fight with each other and have been put in those fights from the get go. Of course the story must be done well with well done presentation and such or else it won't work. That is why NRS' story modes are the best on the market, because they know how to make them right. Now if you're one of those who pick a char based on their gameplan alone that is perfectly fine, but a lot of people pick characters for the story, whatever it's their main reason, one of the reasons or even the main reasons why they did such, and backstory can be showcased via the story mode of a fighting game.