I'm no stranger to games like Hero or other improv-based or acting-slanted games. I highly enjoy them.
However, I have also played a lot of games with a diverse number of people, many of whom I would call casual at best. For these gamers, it is difficult to come up with stuff on the fly, or to reskin things. Because it's difficult at first, many don't "figure it out" for several sessions, sometimes the entirety of the campaign. For others, they are good at it, but they need something to start them off, and the text helps them with that.
In my experience, and this is purely an anecdote, there are more gamers of THIS type then there are gamers of our type. I can come up with a cool image or thematic at any point for any spell. You probably can too. This is not an ability the general public or general D&D-player has, and it isn't easy for them to gain this ability. Because of that, in order to help stimulate creativity, I'm a big fan of flavor text. Give me some flavor. Some spice. Let the flavor teach my players and many other casuals how to imagine Fantasy so that one day, if they want to, they can create Fantasy whole cloth.