Many artists use instagram to display their art, and a big portion of that art is digital and it was intended to be perceived on the screen, so it can’t be collected/purchased in the traditional sense. That’s why a lot of artists I follow and deeply respect were supporting this concept - finally their 3d renders, digital images etc. can enter the art market. See @mikeyjoyce
It is like a democratization of collecting art, instagram being the medium. The motivation for collecting digital art being the same as collecting physical artworks (paintings, sculptures, whatever).
On the other hand, it gives you an opportunity to support the artist you like and get something in return, in a similar fashion bandcamp does with music.