#3 Dani Alves has the Barcelona DNA

Barcelona lost something on the day Lionel Messi left them to sign for Paris Saint Germain. They did not just lose the player but also his goals. Messi was a big part of Barcelona, and he was intricately linked with the identity of the club.
It may forever be tied with him. But it is one thing to have him retire and another thing entirely for him to play for another club just a 1000 miles away. In losing Messi, Barcelona lost something intangible.
That is why getting someone like Dani Alves is a small step in trying to reclaim that. This is a new board in charge of Barcelona. With a club legend like Xavi now managing them, they are trying to establish some connection with the fans who fondly remember and yearn for those old days.
Getting Dani Alves helps achieves that by giving the fans something familiar, something the fans are used to and something they were very fond of. Just having Alves at the Camp Nou creates an atmosphere of nostalgia, and keeps the fans engaged.
#4 Dani Alves could still be an impact super sub

At 38, Dani Alves is older than most players, but he is not old. Sure, he might not be able to play 90 minutes every week like he used to, but he can still come in and put in a sharp cameo or two.
It is not unheard of. Didier Drogba did it with Chelsea on his return to the club, and Juventus used Gianluigi Buffon on and off. It would be impractical to have Dani Alves at your disposal and not make use of his talents. I mean, he is right there.
Moreover, the advantage with the Brazilian is that he can functionally play anywhere on the pitch - defence, midfield, even offence if it comes to that. With La Liga allowing five substitutions now, fans might see more of Alves than they would expect to.