Because it not only somewhat justifies Mabel’s obsessive clingyness, but actually displays her in a light ehere she is capable of actual affections. In the Show Mable’s “romances” were extremely shallow, based more on how cute the boys was and wheter he would do stereotypcial Disney channel boyfriend things with her than anything else. It’s pretty obvious that she doesn’t know anything what actual affection is like as when she wanted to break up with Gideon, she sent her brother to do it for her, because it would make her uncomfortable, which regardless of Gideon’s sanity is a dick move.
And speakig of her brother, her realtionship with him is also messed up. It’s less about how she likes having him around and more that she likes having him around to save her ass, which she almost never reciprocates. She doesnt actually value her brother, she just values the idea of having a twin brother, and she even proves it with Dippy Fresh, who is nothing like Dipper.
But in Pinecest land, Mable actually does love her brother. Granted, it’s a different and much less wholesome kind of love, but still a real one that she would do anything for. And the idea of Little shit Mable leaning and being able to Love someone and not just how they can be useful to her is sweet.