I am not on Instagram, but I was looking into opening one recently.

For an artists perspective its *free advertising for your art. Easy way to get an audience and sell your wares. Also companies may sponsor you or send you stuff to advertise.

In my industry (animation) people have gotten hired through their Instagram. Hollywood is a shitshow so they have been trying to get well known artists to "work" on their projects in hopes to get their followers interested.

So mostly its advertising/money.

*free as in you give up the rights to your images posted there. Lots of artists don't understand this and then get upset their work was sold to some LLM.