Futayuri is futa that has the same feeling as yuri. Yuri, that happens to have a penis on one of the girls.
Regular Futanari is more rough in nature, its a male view of sex, of domination and deviance. Futayuri is often more soft in nature, its like yuri.
There's a lot of purposes of futa in general
One of those purposes is that you want to see two girls go at it, but toys just doesn't do it for you, toys are impersonal, toys aren't part of the body, its like you're fucking her with someone elses dick, its just a poor imitation of the real thing, and you also get no feeling from sicssoring. You want to see them have sex, but lesbian sex just doesn't feel complete, there's no real insertion or release, and you don't want to look at or see a man. No you want to see these two girls do it, but without any of the bullshit. You want to see these two girls do it, not this girl and a blankslate faceless male that you're supposed to "identify with" but really you don't because he's not actually a character. You want these two girls to do it, but skip all the bullshit and just give one of them a dick. That's the smallest change you can make while still retaining all the good aspects.
Of course there are dozens of other reasons why futa is popular, that's merely one of the theories in play that makes it popular. Some people like it for the domination and deviance aspect, some like it for the taboo, some are into futa on male, some are into futa on futa, its a niche that has a lot of aspects.