They obviously were trying to find and capture Pence and any available Congressperson. Pelosi and AOC were specifically targeted by the mob.
What were they going to do once they captured them?
Killing them was a possibility.
This is where the constitution gives light to why they targeted Mike Pence.
Article II / 12th Amendment
The President of the Senate shall, in the presence of the Senate and House of Representatives, open all the certificates and the votes shall then be counted;
Article I
The Vice President of the United States shall be President of the Senate, but shall have no vote, unless they be equally divided.
This means that ONLY the vice president can open and count the votes. Without Pence, there could legally be no count.
Somebody would have to rule whether the president pro-temp of the senate, can serve as president of the senate for constitutional purposes.