what was Reuben Tishkoffs role?

I believe he was the one who financed everything. Paid for what needed to be paid for. Trucks, uniform, equipment, the vault, etc.

Seize the moment, 'cause tomorrow you might be dead.

Yep, he was their backer. Funded it all

Mostly financial, but he was also the only person in the group who could be close to Benedict without raising any suspicion. He was on the stage for the implosion, and he was sitting behind Benedict at the fight. This way the group would have a way of making sure Benedict was acting and reacting to things exactly the way they needed him too.

He was arguably the most essential team member. Without him, they were screwed.