BB does this dome task whenever they wanna get rid of a particular contestant: on BB11 they nominated only Luv & Priyank hoping Luv would get evicted. But instead Priyank was gone as Luv developed massive fan following + community support.
This season they did this task to get rid of Arti. How? They nominated Arti along with Sid, hence ensuring Sid fans can't vote for her. They also saved Rashmi-Asim from nomination, hence making sure their fans can throw some votes for Vishal so that he gets safe.
Why not just safe Vishal? That would look fishy if all strong contestants are only nominated along with Arti. That will also ruin the excitement of who goes home as everyone, including Arti would know she's gone.
They kept Arti around for family tasks, as they had plans to bring Krushna & Kashmera which is now fulfilled, hence Arti's job is over for BB team. Arti will likely go this week.